Added support for Python 3.1+

- Introduces the six library as a dependency and places the used modules
into pyleus.compat.
- Some changes to how objects are mocked to keep things compatible with
Py2 and Py3 versions. Nested context managers had to be changed since 
2.6 doesn’t support the multiple managers in one with stmt and 3.1+ drops
support for the contextlib.nested manager.
- Updated tox.ini and all tests are passing for py26, py27, py31, py32, 
py33, and py34.

The tests are working for me now so as long as the tests cover the code well then everything should work.

Addresses #15 and #37
Could you move also the import of `builtins` to `pyleus.testing`?