Create shared namespaces for powerline.{matchers,segments}.plugin
Add NERDTree statusline
Add Control-P statuslines

This hacks around the way CtrlP handles statuslines. Powerline will
always override the statusline, but CtrlP doesn't make its details
available as buffer variables so separate functions assign the details
to buffer variables each statusline redraw.

Currently this statusline only uses a couple of highlight groups,
colorschemes should add the HL groups to make this prettier.
Add error/warning HL groups for vim
Align default vim colorscheme attributes
Add Syntastic statusline

Refs #376, #639.
Closes #451.
Update plugin segments to use bindings
Update tests

Unit tests are not run on external segments. When external segments are
moved out of the core repo the extra test code won't be needed.
Update CtrlP config to match segment changes
Add Gundo statuslines
Center align CtrlP current segment
Add Tagbar current tag segment
Merge branch 'feature/plugin-statuslines' into develop

Refs #3, #524, #470
Closes #377
Closes #451