Add missing import

Also add tests

Fixes #511

powerline/bindings/vim/plugin/powerline.vim

@@ -40,5 +40,6 @@ finally
 	endif
 endtry
 
+execute s:pycmd 'import vim'
 execute s:pycmd 'powerline_setup(pyeval=vim.eval("s:pyeval"), pycmd=vim.eval("s:pycmd"))'
 execute s:pycmd 'del powerline_setup'

tests/test.sh

@@ -10,4 +10,7 @@ done
 if ! ${PYTHON} scripts/powerline-lint -p powerline/config_files ; then
 	FAILED=1
 fi
+if ! vim -S tests/test_plugin_file.vim ; then
+	FAILED=1
+fi
 exit $FAILED

tests/test_plugin_file.vim

@@ -0,0 +1,16 @@
+#!/usr/bin/vim -S
+set nocompatible
+try
+	source powerline/bindings/vim/plugin/powerline.vim
+catch
+	cquit
+endtry
+set ls=2
+redrawstatus!
+redir =>mes
+	messages
+redir END
+if len(split(mes, "\n"))>1
+	cquit
+endif
+quit!