Add helm-projectile-grep/ack/ag  and update documentation

Add replacement commands for projectile-grep, projectile-ack and
projectile-ag. When a symbol is at point, use that symbol and search at
project root. If a region is active, use the region instead (only work
with helm-projectile-grep/ack).

DEMO:

![helm-projectile-grep](https://cloud.githubusercontent.com/assets/4818719/4879969/4ce53926-6334-11e4-9429-bf60636396c3.gif)