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.

As next release will not be pure bugfix any more (we got additional
options), we increase the minor version number.

The new option is not active yet.

The sources can be registered (as entry_points) in some arbitrary
module available at runtime. We register some modules ourselves,
'system' for starters.

Yet we do not make use of the registered sources in main(). This
will be the next step.

We will use a 'diceware_random_sources' group to look for random sources registered in any Python package in a running environment. We start with the default source (deploying `random.SystemRandom`).

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.