Massive Generator Refactoring

* I've created a generator factory class and a generator interface.
* I've deleted the call generator, and turned the closure generator into a callable generator (php 5.4+).
* I've deleted the generic generator (now we use the new Faker facade, with the callable generator).
* I've kept the readme up-to-date with the changes, but the upgrading guide is not filled out yet.
* I've ditched Zizaco's name from our license headers for v3.0 since none of the code is his now (his name will remain on the readme as the founder though).
* There are been a few class renamings too.

This actually required minimal changes to our tests, which is pleasing.