Remove speedment()-method from entities

[Pyknic]

Having abstract entities that are implemented anonymously by the manager is not a very good pattern. It results in more complex design and makes it more difficult to understand how Speedment works. The only reason for this is that we don't want to store the Speedment instance in every entity and we need it to persist, update and delete entities.

A potential solution for this would be to include the Speedment-instance in the method call where it is needed. It would look something like this:

User user = new UserImpl()
    .setFirstname("Foo")
    .setLastname("Bar")
    .persist(speedment);

This would be quite a big API change and would therefore have to wait until the 2.4.0 Forest release.