Refactor cwd segment into a class and add shorten_home argument
Make powerline autodoc add all Segments
Move with_docstring function to powerline.segments
Add support for shell-specific path shortening

Fixes #502
Do not specify $PYTHON when running python client

It is not needed: in the environment where it is run only python is the one
linked in tests/shell/path.
Merge pull request #1014 from ZyX-I/zsh-named-dirs

Add support for zsh named directories