Escape shell commands on windows.
Fixed escaping
Now working with shellslash
Merge branch 'issue-84'

* issue-84:
  Now working with shellslash
  Fixed escaping
  Escape shell commands on windows.
Update signs in realtime.

Thanks to @ashb for the help with this.
Update help text.
introduce GitGutterGetHunkSummary for statusline integration.
extract updating hunk summary into another function.
add documentation for GitGutterGetHunkSummary.
Move hunk summary function to diff utility section.