helm-projectile.el: rewrite `helm-projectile-define-key' to allow defining multiple keys

Rewrite `helm-projectile-define-key' to allow defining multiple keys. Then use
the enhanced macro to cleanup the code where `helm-projectile-define-key' are
being called repeatedly while defining multiple key bindings.

To illustrate, previously we have to write:

(helm-projectile-define-key KEYMAP KEY1 DEF1)
(helm-projectile-define-key KEYMAP KEY2 DEF2)
(helm-projectile-define-key KEYMAP KEY3 DEF3)

Now, we can clean it up to:

(helm-projectile-define-key KEYMAP
  KEY1 DEF1
  KEY2 DEF2
  KEY3 DEF3)
Merge pull request #470 from YorkZ/helm-switch-project

helm-projectile.el: rewrite `helm-projectile-define-key' to allow defini...