Make 'powerline' a namespace package

This will allow third-party plugins to use the 'powerline' namespace for
contributed segments.

Refs #3.
Merge branch 'develop' into zsh/zpython
Made zsh bindings restore prompt

Prompt is restored when zsh/zpython module is unloaded: as ZPYTHON_* variable is 
readonly (unless __call__ method is defined) it is currently impossible to 
delete it without unloading module, though prompt can be restored independent of 
variable deletion. This zsh/zpython behavior is currently considered to be 
a feature. If __call__ method is defined prompt will also be restored when 
variable is unset