Added file_vcs_status segment, using powerline.vcs instead of fugitive

Using pure-python implementation allows to use the same code in the terminal 
segments (not implemented currently). It also should be more effective. Git 
module is able to use pygit2 instead of command-line git which is much faster. 
Mercurial module assumes that mercurial is always accessible as a python module.

Code for repository (not file) status is untested. It exists solely for terminal 
segments.