Features in @Entity and @Flush. Solves #11 #7

* Changes in @Entity
    * Entity can be persisted.
    * default_manager can be overwritten in bundle parameters
    * manager can be defined in annotation
    * default_persist can be overwritten in bundle parameters
    * persist can be defined in annotation
    * Tested
* Change in @Flush
    * entity can be defined in annotation
    * if empty, flushes all
    * if single value, flushes single entity
    * if array, flushes all entities in array
    * Tested
* Updated readme
* Some funcional tests. Just entity one
Merge pull request #12 from mmoreram/flush-specific-entities

Features in @Entity and @Flush. Solves #11 #7