Migration to Play 2.3 - second part

Hi !

Here is the second part of the migration to the version 2.3 of the Play framework (first part was PR #44). This second PR addresses remarks in this [comment](https://github.com/hakandilek/play2-crud/pull/44#issuecomment-51668203).

PR includes especially:
* use of webjars
* projects which use play2-crud are able to use custom assets defined in play2-crud
* fix `value RouterCrudController is not a member of object play.utils.crud.routes` (cancel 4de42bf)