1. solve the hanging problem if the retrieved object is not an valid type T.
2. add bson object id exception handling
Merge pull request #10 from tianjianfeng/issue-9

fix issues 3, 4, 5
Merge commit 'fc3381a99003b03562b209a45621edc1d1350aba'

Conflicts:
	app/controllers/UserController.scala
	app/models/User.scala
	app/services/DBServiceComponent.scala
	test/unit/controllers/UserControllerUnitSpec.scala