The new real dice random source now works on a very plain level.

Includes an InputMock to fake keyboard input. With `InputMock` we
can fake std lib `input()`/`raw_input()` using py.tests
'monkeypatch' fixture.

This will only be a skeleton, to be filled later.

We got an `input_func` which works regardless of Python version (as `raw_input`
was renamed `input` with Python 3.

We also got a RealDiceRandomSource class without any functionality
yet. The `get_input` method is only a temporary placeholder to enable
testing of mocking the input_func.

Based on suggestions of @drebs we now start to support different
sources of randomness. We start with one source, the default,
which will be the standard libs `random.SystemRandom` source
we used anyway. But from now on this source if put into a
separate class, tested and all that, which will be registered
as an entry point.

Yet we do not register or work with the registered sources. This
will come next.