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.

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.

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.

The former default ``1`` prevented differed from handle_options()
default (``0``). We are back at 100% test coverage now.

We now use an `argparser` options instance for get_passphrase
instead of a bunch of single args. This makes the main function
`get_passphrase` less readable (you do not see immediately what
options are really expected), but avoids misleading defaults. We
now use the defaults set in argparser options, i.e. from
`handle_options()` as single source of defaults.

We now read from already opened file descriptors (in get_wordlist()) only.