Workaround Python import conflicts with sys.path inside Vim

After updating Vim to version 7.3.1163, Powerline refused to run its setup routine. 

```An error occurred while importing the Powerline package. This could be caused by an invalid sys.path setting, or by[...]```

Since my Powerline sources are not in any common Python package directory, loading depends on sys.path modification. Actually sys.path contains '_vim_path_' value. I don't know if it's new, but it seems to me that module lookup is not done inside subsequent items (directories).

```sys.path = ['/usr/lib64/python27.zip', '/usr/lib64/python2.7', '/usr/lib64/python2.7/plat-linux2', '/usr/lib64/python2.7/lib-tk', ..., '_vim_path_', '/home/covin/.vim/bundle/powerline']```

Prepending Powerline directory to sys.path fixes loading problem for me:

```sys.path = ['/home/covin/.vim/bundle/powerline', '/usr/lib64/python27.zip', ..., '_vim_path_']```