Add stats property to g:sy
Merge pull request 58 from killphi
New function: sy#repo#get_stats()

It returns a list of numbers of additions, modifications and deletions
and is intended to be used by other plugins.
Fix problem on :SignifyToggleHighlight

It was possible to run the command before the variable
signify_line_highlight was set. It's disabled by default now.

Closes #59.
Reset stats when the plugin is toggled off
Change init stats for disabled plugin