Work around missing vim module when generating sphinx docs

Horrible solution, should be fixed asap.

powerline/bindings/vim/__init__.py

@@ -1,6 +1,9 @@
 # -*- coding: utf-8 -*-
 
-import vim
+try:
+	import vim
+except ImportError:
+	vim = {}
 
 try:
 	_vim_globals = vim.bindeval('g:')

powerline/segments/vim.py

@@ -3,7 +3,10 @@
 from __future__ import absolute_import
 
 import os
-import vim
+try:
+	import vim
+except ImportError:
+	vim = {}
 
 from powerline.bindings.vim import vim_get_func
 from powerline.lib import memoize, humanize_bytes