entity_for accepts an entity class.

Defaults to the mapper's entity class.
Safer removal of 'Mapper' suffix.
Memory -> Mapper::Memory.
Whitespace.
Common -> Mapper::Common
AR -> Mapper::AR.
Comments.
Cleanup.
Entities have mapper_errors.

Not yet used. Will be used for e.g. uniqueness validations.
Copy uniqueness validations from record to entity.
Bump version.
Merge pull request #4 from barsoom/master

Import barsoom improvements.