Make sure it is shown only if neccessary (and not shown otherwise).

Fix real dice input range validation

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.