Support for pinning package to archive

Emacs 24.4 support pinning a package to a certain archive (gnu, melpa, melpa-stable, etc) and now use-package can handle that as well:
````elisp
(use-package alchemist
  :ensure t
  :pin "melpa-stable")
````
We have to call `(package-install)` from `use-package-pin-package` after touching `package-pinning-packages` which is a bit slow.