First pass at large restructuring

Hi Edwin,

This is a first pass at adding some more structure to the PyGaze codebase. Essentially, all classes have been systematically split up across separate files and organized into separate packages. I think this was necessary to make the code more readable and maintainable, and also to add extra functionality later on (notably integration with OpenSesame).

There are some changes in the API, but everything should be backwards compatible as well. For example, you can now say ...

~~~ .python
from pygaze import Display, Screen
~~~

... but the old style still works as well ...

~~~ .python
from pygaze.libscreen import Display, Screen
~~~

That being said, I'm pretty sure that I broke *a lot* of stuff with this reorganization, particularly the `eyetracker` stuff, which I can't test from home. These things should be easy to fix though. So don't consider this a fully functional and polished pull request: I just wanted to let you know already to avoid working on completely different branches that will be difficult to merge back together later on.

Cheers,
Sebastiaan