Make discriminator digit-aware

[johncf]

Fixes #17

Implements a solution very similar to the one proposed in the issue:

• "Fake" input: the one-hot representation of the label + the Encoder's style encoding output.
• "Real" input: the one-hot representation of the label + a prior-distribution random-sampled style vector.
• The first part of fake and real inputs are the same for each mini-batch.

Now, everything is great!