Implement basic Mongo entity and utils

- Base mongo-compatible entity using ObjectId values for id.
- utility for converting from mongo document to mongo-compatible entity.
- utility for converting from mongo-compatible entity to mongo document.
Remove unecessary methods from base mongo entity.

- Simplify entity tests.
Delete some files because they are too tightly tied to mongo.
Use a more standard variable name for the class argument.
Merge pull request #1 from jong/feature/mongo-improvements

Feature/mongo improvements