Handle gitfiles when retrieving branch name

Move code to parse a .git file from the do_status method to a new method
shared with get_branch_name so that branch name can be retreived from
repositories which use a .git file.

Closes #634.
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.
Option to suppress error in python-less vim

If the g:powerline_no_python_error variable is set don't issue an error
message even if vim doesn't have the required Python support.  This
allows a common set of configuration files to be used across systems
where some of the copies of vim don't meet the requirements.
ignore trailing whitespace errors in .rst files

Use a .gitattributes file to tell git to not highlight trailing
whitespace as an error in .rst files when showing diffs.  A trailing
space is used on many lines in those files to indicate that the
paragraph continues onto the next line.
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.
Merge remote-tracking branch 'aschrab/no_python_warning' into develop
Merge remote-tracking branch 'aschrab/gitfile' into develop
Merge remote-tracking branch 'aschrab/trailing_whitespace' into develop
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
Remove long-running segments from tmux

The segments will be re-added when the daemon is merged and the docs
are updated.