Merge pull request #504 from tuhdo/master

Add helm-projectile-grep/ack/ag, helm-projectile-commander update documentation
Fix ack binary check and update docstring
Merge pull request #513 from tuhdo/helm-projectile-ack

Fix ack binary check and update docstring
Autoload projectile-kill-buffers
Merge pull request #517 from purcell/patch-1

Autoload projectile-kill-buffers
Make projectile-replace's param optional
Merge pull request #518 from tuhdo/master

Fix helm-projectile-find-dir