Save branch head to a buffer variable

Save branch head to a buffer-local variable to prevent looking up the VCS head for every status line refresh.
Add CursorHold autocmd to unset airline_head
Merge pull request #469 from russellhancox/patch-1

Save branch head to a buffer variable
remove usage of fugitive#detect as per request

https://github.com/tpope/vim-fugitive/issues/463
restore original directory change detection. also resolves #474.
Now trailing tabs are also reported, fixed bug when trailings with mixed tabs and spaces were being reported as mixed indents, case when count of spaces at the end of tabbed indentation less then tabstop does not reported as mixed indent anymore.
Merge pull request #478 from davits/mixed_indent

Improvements in whitespace plugin
handle cases where the augroup has not be defined yet. resolves #492
add support for configurable whitespace checking algorithms and default it to the simplest one. resolves #484
provide option to disable runtimepath extension loading. resolves #483
Enable :AirlineToggleWhitespace,

even if g:airline#extensions#whitespace#enabled is disabled
Merge pull request #493 from chrisbra/master

Enable :AirlineToggleWhitespace,
append to the existing value in case the user defined it.
new NrrwRgn integration
doesn't assume user has 'set autochdir' set
Merge pull request #496 from chrisbra/new_feature_nrrwrgn

new NrrwRgn integration
Merge pull request #497 from johnathanz/master

doesn't assume user has 'set autochdir' set
fix minor typo in help text
Merge pull request #498 from lumbric/master

fix minor typo in help text
Update README.md vundle installation instructions

[vundle is undergoing interface changes](https://github.com/gmarik/Vundle.vim/blob/master/doc/vundle.txt#L372-L396)