Add projectile-vc-dir bound to C-c p v

Define new function projectile-vc-dir which runs vc-dir on the project's root directory.  Bind this to C-c p v by default.
Add C-c p v (projectile-vc-dir) to Keybinding summary
Merge pull request #198 from nwidger/master

Add projectile-vc-dir bound to C-c p v