DATAMONGO-709 - Added support for restricting results by document types.

Added restrict(…) method to the Query API that generates appropriate filter criteria to restrict the result to certain types only. 

Type restrictions in query expressions are now applied in QueryMapper via a MongoTypeMapper based on information passed in through the query object in a "special" key. Exposed MongoTypeMapper in MongoConverter and MappingMongoConverter. Merged DefaultTypeMapper and DefaultMongoTypeMapper.

Original pull request: #53.