Use better name for powerline variable in global scope

It is completely possible that “pl” will be claimed by some other plugin
as well. There is no local scope for each .vim file in vim-python
interface.

powerline/ext/vim/powerline.vim

@@ -1,18 +1,18 @@
-python import uuid, vim
+python import uuid
 python from powerline.core import Powerline
-python pl = Powerline('vim')
+python powerline = Powerline('vim')
 
 if exists('*pyeval')
 	let s:pyeval = function('pyeval')
 else
-	python import json
+	python import json, vim
 	function! s:pyeval(e)
 		python vim.command('return ' + json.dumps(eval(vim.eval('a:e'))))
 	endfunction
 endif
 
 function! Powerline(winnr, current)
-	return s:pyeval('pl.renderer.render('. a:winnr .', '. a:current .')')
+	return s:pyeval('powerline.renderer.render('. a:winnr .', '. a:current .')')
 endfunction
 
 function! s:UpdateWindows()