DATAMONGO-1054 - Add support for fast insertion via MongoRepository.insert(..).

Introduced new insert(..) method variants on MongoRepositories that delegates to MongoTemplate.insert(..). This bypasses ID-population, save event generation and version checking and allows for fast insertion of bulk data.

Original pull request: #253.
DATAMONGO-1054 - Polishing.

Tweaked JavaDoc of the APIs to be less specific about implementation internals and rather point to the save(…) methods. Changed SimpleMongoRepository.save(…) methods to inspect the given entity/entities and use the optimized insert(All)-calls if all entities are considered new.

Original pull request: #253.