Adds HeadAction class to play.api.mvc.controllers. Closes #2280

* This is an RFC2616 compliant impementation of the HEAD verb (Provides headers for a GET route)

* Adds HttpVerbs object to StandardValues, providing non-MagicStrings lookup of GET/POST/HEAD/etc.

* Update "GET" and "HEAD" references in ContentTypes to use HttpVerbs instead
Merge pull request #2355 from Damiya/feature/head_action

Adds HeadAction class to play.api.mvc.controllers. Closes #2280