Merge pull request #35: Create session for current tab page only

I ported Ingo's code (based on vim-session 1.5) to vim-session
1.8 and changed quite a few things in the process:

 - Most significantly, in my version :OpenSession, :SaveSession
   and :CloseSession keep their original meaning and the commands
   :OpenTabSession, :SaveTabSession and :CloseTabSession should be
   used to manage tab scoped sessions.
 - Added documentation
 - Made periodic auto-save respect tab scoped sessions
 - Made lock files aware of tab scoped sessions and process IDs
 - Replaced set_session_name() with t:this_session / v:this_session
 - Auto-load functions for tab scoped commands,
   for easy re-use in command aliases
 - Renamed PushTabSessionOptions() to change_tab_options()
 - Renamed PopTabSessionOptions() to restore_tab_options()
 - Reimplemented session_name() as find_current_session()
 - Reimplemented is_tabsession() as is_tab_scoped()
 - xolox#session#save_session() now rewrites 'doautoall'
   in tab scoped sessions to 'windo doautocmd'

Author: xolox

README.md

@@ -72,8 +72,34 @@ Note that this command only deletes the session script, it leaves your open tab
 
 Execute this command to view the Vim script generated for a session. This command is useful when you need to review the generated Vim script repeatedly, for example while debugging or modifying the `session.vim` plug-in.
 
+### Tab scoped sessions
+
+When ['sessionoptions'] [sessionoptions] contains 'tabpages' (this is the default) session scripts will persist and restore all windows in all tab pages. When you remove 'tabpages' from ['sessionoptions'] [sessionoptions] you get a sort of light-weight sessions: They are constrained to a single tab page. Vim's [:mksession] [mksession] command and the vim-session plug-in both fully support this.
+
+You can change ['sessionoptions'] [sessionoptions] in your [vimrc script] [vimrc] but then you can never save a session including tab pages. To decide on the spot whether you want a global or tab scoped session, the vim-session plug-in defines the three commands documented below.
+
+Note that tab scoped sessions are regular session scripts, so when you load a tab scoped session using `:OpenSession` instead of `:OpenTabSession` the vim-session plug-in *assumes* it is a global session and will close all active tab pages before opening the tab scoped session.
+
+#### The `:OpenTabSession` command
+
+Just like `:OpenSession` but applies only to the current tab page.
+
+#### The `:SaveTabSession` command
+
+Just like `:SaveSession` but applies only to the current tab page.
+
+#### The `:AppendTabSession` command
+
+This command opens a new tab page and loads the given tab scoped session in that tab page. You can give this command a count just like [:tabnew] [tabnew].
+
+#### The `:CloseTabSession` command
+
+Just like `:CloseSession` but applies only to the current tab page.
+
 ## Options
 
+The following Vim options and plug-in options (global variables) can be used to configure the plug-in to your preferences.
+
 ### The `sessionoptions` setting
 
 Because the `session.vim` plug-in uses Vim's [:mksession][mksession] command you can change how it works by setting ['sessionoptions'][sessionoptions] in your [vimrc script] [vimrc], for example:
@@ -166,6 +192,13 @@ When this option is enabled the session plug-in will define the following comman
  * `SessionDelete` is an alias for `DeleteSession`
  * `SessionClose` is an alias for `CloseSession`
 
+Then there are the command aliases for tab scoped sessions:
+
+ * `SessionTabOpen` is an alias for `OpenTabSession`
+ * `SessionTabSave` is an alias for `SaveTabSession`
+ * `SessionTabAppend` is an alias for `AppendTabSession`
+ * `SessionTabClose` is an alias for `CloseTabSession`
+
 The aliases support tab completion just like the real commands; they're exactly the same except for the names.
 
 When you enable the aliases, the default command names will still be available. If you really don't like them, feel free to delete them using [:delcommand] [delcommand].
@@ -301,4 +334,5 @@ Here's an example session script generated by the `session.vim` plug-in while I
 [mksession]: http://vimdoc.sourceforge.net/htmldoc/starting.html#:mksession
 [sessionoptions]: http://vimdoc.sourceforge.net/htmldoc/options.html#%27sessionoptions%27
 [source]: http://vimdoc.sourceforge.net/htmldoc/repeat.html#:source
+[tabnew]: http://vimdoc.sourceforge.net/htmldoc/tabpage.html#:tabnew
 [vimrc]: http://vimdoc.sourceforge.net/htmldoc/starting.html#vimrc