Fix example in CONTRIBUTING.rst
Use powerline.lib.encoding* in place of sys.get*encoding/locale.get*

Fixes #1078
Pop first path from sys.path

According to the `sys.path` documentation “As initialized upon program startup,
the first item of this list, path[0], is the directory containing the script
that was used to invoke the Python interpreter.” Since client is named
`powerline.py` it tries to use the client itself as powerline module and
obviously fails to import `lib` submodule.