Add sy#util#run_in_dir to run system() within a specific directory

Signed-off-by: James McCoy <vega.james@gmail.com>
Replace all uses of "cd … && do_something" with sy#util#run_in_dir

Signed-off-by: James McCoy <vega.james@gmail.com>
Merge branch 'UNC-paths'

Closes #114
Fix locale setting for Chinese Windows versions

On the Chinese version of Windows, v:lang is "zh_CN", but execute
":language zh_CN" will fail and ":language chinese_china" must be used
to change the locale instead.

Closes #116.
Minor grammar changes in README.md
Merge pull request #123 from sahands/master

Minor grammar changes in README.md
doc/signify.txt: Add perforce to supported VCS
Fix error when toggling on a new file

Toggling on a new file lead to an error since b:sy wouldn't exist at
that time.

Closes #124.
Perforce: check for server availability before requesting a diff

References #131.
Doc: add possible values for g:signify_vcs_list

References #132.
Doc: change tags to common conventions
Improve :SyDebug
Fix help filetype skipping
Skip help files by default
Introduce smarter sign management
README: update
Make sure b:sy_path is always set

References #136.
Introduce hunk text object
Doc: add remark about ]c and [c
Execute FocusGained action for all windows

References #113.