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.
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.
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