Defer autocmd creation until all plugins are loaded

This update ensures that Powerline's autocmds aren't created until vim
has loaded all plugins and settings, to avoid errors with plugins that
are loaded after Powerline. The VimEnter event is used to accomplish
this.

Closes #152.
Refs preservim/tagbar#77.

plugin/Powerline.vim

@@ -42,20 +42,28 @@
 	endif
 " }}}
 " Autocommands {{{
-	augroup Powerline
-		autocmd!
+	function! s:CreateAutocmds()
+		augroup PowerlineMain
+			autocmd!
+
+			" Reload statuslines when changing color scheme
+			autocmd ColorScheme *
+				\ call Pl#Load()
 
-		" Reload statuslines when changing color scheme
-		au ColorScheme *
-			\ call Pl#Load()
+			autocmd BufEnter,WinEnter,FileType,BufUnload *
+				\ call Pl#UpdateStatusline(1)
 
-		au BufEnter,WinEnter,FileType,BufUnload *
-			\ call Pl#UpdateStatusline(1)
+			autocmd BufLeave,WinLeave *
+				\ call Pl#UpdateStatusline(0)
 
-		au BufLeave,WinLeave *
-			\ call Pl#UpdateStatusline(0)
+			autocmd BufWritePost */autoload/Powerline/Colorschemes/*.vim
+				\ :PowerlineReloadColorscheme
+		augroup END
+	endfunction
+
+	augroup PowerlineStartup
+		autocmd!
 
-		au BufWritePost */autoload/Powerline/Colorschemes/*.vim
-			\ :PowerlineReloadColorscheme
+		autocmd VimEnter * call s:CreateAutocmds() | call Pl#UpdateStatusline(1)
 	augroup END
 " }}}