Add Helm action for incremental grep in selected projects

Add a new Helm action (bound to "C-s") to do incremental grep in the selected
projects. To use it, press "C-c ph" to fire `helm-projectile', select your
project (press "C-SPC" to select multiple projects), type "C-s" (or "C-u C-s"
for recursive grep), and then type your regexp. As you type the regexp in the
mini buffer, the live grep results will be displayed incrementally.
Merge pull request #428 from YorkZ/helm-switch-project

Add Helm action for incremental grep in selected projects