Move plugin loading with :source into separate file

This should resolve the issue with duplicate paths in sys.path by only
updating sys.path if the plugin is loaded using :source instead of
:python. sys.path is now updated in source_plugin.vim, which checks if
the user has Python installed, and updates sys.path before sourcing the
actual plugin file.

Refs #17.
Refs #19.
Refs #21.

docs/source/overview.rst

@@ -64,4 +64,4 @@ location.
 Add the following line to your ``vimrc``, where ``{path}`` is the path to 
 the main Powerline project directory::
 
-    source {path}/powerline/ext/vim/powerline.vim
+    source {path}/powerline/ext/vim/source_plugin.vim

powerline/ext/vim/__init__.py

@@ -1,7 +1,8 @@
 # -*- coding: utf-8 -*-
 
+
 def source_plugin():
 	import os
 	import vim
 
-	vim.command('source ' + os.path.join(os.path.abspath(os.path.dirname(__file__)), 'powerline.vim'))
+	vim.command('source ' + vim.eval('fnameescape("' + os.path.join(os.path.abspath(os.path.dirname(__file__)), 'powerline.vim') + '")'))

powerline/ext/vim/powerline.vim

@@ -1,13 +1,4 @@
-if ! has('python')
-	echohl ErrorMsg
-	echomsg 'You need vim compiled with Python 2 support for Powerline to work. Please consult the documentation for more details.'
-	echohl None
-	finish
-endif
-
-python import sys, vim, os
-python sys.path.append(vim.eval('expand("<sfile>:h:h:h:h")'))
-python import uuid
+python import uuid, vim
 python from powerline.core import Powerline
 python pl = Powerline('vim')
 

powerline/ext/vim/source_plugin.vim

@@ -0,0 +1,11 @@
+if ! has('python')
+	echohl ErrorMsg
+	echomsg 'You need vim compiled with Python 2 support for Powerline to work. Please consult the documentation for more details.'
+	echohl None
+	finish
+endif
+
+python import sys, vim
+python sys.path.append(vim.eval('expand("<sfile>:h:h:h:h")'))
+
+exec 'source '. fnameescape(expand('<sfile>:h') . '/powerline.vim')