Message module does not follow the Entity API interface

Problem/Motivation

Already described here: https://github.com/Gizra/message/issues/149
Opening the issue for visibility purposes.

Proposed resolution

See PR on github: https://github.com/Gizra/message/pull/150

Remaining tasks

Approval from maintainers, commit etc...

User interface changes

None.

API changes

API addition (backwards compatible one).

Data model changes

None.