Fixing an issue occuring with php 5.3

Implementing interfaces methods as abstracts in an abstract class doesn't work with PHP 5.3 (it does with 5.4 however). Removing those methods abstract implementations fixes this.
Added PHP 5.5 to .travis.yml
Use interface constant as query parameter.

Message class does not exist in this scope, so the cleanup command ends in a fatal PHP error.
Add unit tests for MessageManagerIterator & fix bug with doctrine context and command optimize method