Switch to using ivy cache in users home directory

One small step for Play, one giant leap back to using SBT in a vanilla
configuration...

* Adds the play local repository to the list of SBT boot repositories,
  so it gets automatically used like it used to when it was instead the
  default local repository.
* Makes the ivy home directory the default one, in the users home
  directory.
* This means ivy cache is shared between vanilla SBT/ivy apps and Play
  apps.
* Also means local repository (when you run publish-local) is shared.