3 0 compatibility

[josegonzalez]

This pull request implements partial compatibility with the CakePHP 3.0 Entity specification.

• Entity classes now are not wholly tied to the Model class, and have no reference to them
• Model class is now treated as the Table class. Subsequently, missing Table methods have been added, where appropriate
• Removal of helper methods - such as save and updateAll - from the Entity classes. This is because they belong in the Table class (which implements a sort of session) not in the Entity class.
• Removed entity->isAllowed()
• Entity class is now JsonSerializable. Woot
• entity->toArray() returns a nested data structure, as opposed to top-level as in 3.x. For example, 3.x has ['name' => 'Jose', 'email' => 'mark@example.com'], whereas CakeEntity will have ['User' => ['name' => 'Jose', 'email' => 'mark@example.com']]. This is to deal with data saving - we will leave the 2.x saving largely untouched so that the entities can be easily used in the FormHelpers for instance.
• PHP 5.4+ required.

Todo:

• Write tests for data saving
• Implement EntityModel::alias()
• Implement EntityModel::connection()
• Implement EntityModel::association()
• Implement EntityModel::associations()
• Implement Entity::validate()
• Move serialization of entities into a Marshaller class, similar to how 3.x has

[coveralls]

Coverage remained the same when pulling 62ab865 on 3-0-compatibility into 22a0be5 on master.