bumped scala 2.10 and lift 2.5 versions

- Lift 2.5.1 was released, no need to depend on snapshot
- Scala 2.10.2 has been released
ObjectIdKey trait to replace lift MongoId

MongoId is deprecated in Lift 2.5 and that causes problems for codebases
that are heavily reliant on that functionality.
Merge pull request #88 from foursquare/objectidkey

ObjectIdKey trait to replace MongoId