Simplify grass init script (#5)

* shutil.rmtree removes directories recursively and has ignore errors, so no need for the wrapper
function/classes

* lib/init/grass: Simplify encode()/decode() functions.

* lib/init/grass: Move all the imports at the beginning of the file.

* lib/init/grass: Simplify clean_env().

"clean_env()" is only being called once. Even if it gets called multiple
times in the future, it is unlikely that it will be used with a different
"gisrc".

* Just keeping globals and the initialization stuff at the beginning of the
module

* lib/init/grass: Group the logging functions together.

* lib/init/grass: Move the info message out of the "show_info()" function.

This was messing up with the vim code folding.

* lib/init/grass: Remove unused constants

* lib/init/grass: Remove old comment about removed nested session test

The comment is from 2015. If there were problems, they would have surfaced
since then.

* lib/init/grass: Move all the gisbase stuff in a single place.

* lib/init/grass: Capitalize all constants

* lib/init/grass: Simplify setting a value to CONFIG_PROJSHARE

* lib/init/grass: Add comments about GISBASE

* lib/init/grass: Validate cmdline in a separate function

* lib/init/grass: Inline variable substitution inside "help_message()"

The main motivation here is to avoid making "_()" calls at the module
level which will hopefully let us remove the early "gettext.install()"
call in a later commit.

* lib/init/grass: Refuse to start if ~/.grass7 exists but is not a directory

* lib/init/grass: Ensure that the init process only happens inside main()

1. we remove the "gettext.install()" call from the module level
2. we make sure that "set_language()" is the first step of the init process.

* lib/init/grass: Use six for cross version Python compatibility

Six is being pulled by matplotlib anyway, but for good measure,
I also added it to the explicit requirements.

REQUIREMENTS.html

@@ -185,6 +185,10 @@ <h3>Optional packages:</h3>
 <a href="https://pypi.python.org/pypi/termcolor">https://pypi.python.org/pypi/termcolor</a>
 </li>
 
+<li><b>six</b> (needed for cross-version Python compatibility)<br>
+<a href="https://pypi.python.org/pypi/six">https://pypi.python.org/pypi/six</a>
+</li>
+
 <li><b>FFMPEG or alternative</b> (for wxGUI Animation tool - g.gui.module),
     specifically ffmpeg tool<br>
 <a href="http://ffmpeg.org">http://ffmpeg.org</a>

Vagrantfile

@@ -71,6 +71,7 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
       "python-numpy",
       "python-ply",
       "python-pil",
+      "python-six",
       "libnetcdf-dev",
       "netcdf-bin",
       "libblas-dev",