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 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).

This actually required minimal changes to our tests, which is pleasing.

@scottrobertson If you don't get to this before I remove the generic generator, and add a faker generator, then I'll just add them to this pull since it'll be dependant on these changes.