Add Helm interface to switch project

Hi Bozhidar Batsov,

I've just added a new feature to allow switching project using helm interface.
Please note that this is different from simply setting
`projectile-completion-system` to `helm` which only enables projectile to use
the Helm Completion to complete a project name. This commit added new command
`helm-projectile-switch-project`, and bind to "H" in projectile command map by
default. The new command uses helm interface instead of ido to switch project.
The benefit of this approach is that on any selected project we can fire many
actions, not limited to just one action of "switch to project", as in the case
of using helm completion by simply setting `projectile-completion-system` to
`helm`. Currently there are only three actions, namely "Switch to project",
"Open Dired in project's directory" and "Switch to Eshell", but will definitely
add more in the future.

Could you please consider including my patch?
The changelog entry should be more concise. Consider adding more documentation about this somewhere in the README (not sure if we have a section about `helm-projectile` - if we have one you can extend it).