API to specify prior

The user should be given a clean way to specify the prior distribution.

IMHO it should be provided as an optional parameter to GAN constructor, with possible constant values 'gaussian', 'uniform' (and 'UCT' eventually :-) ), which would draw the prior samples from these distributions (just reflecting the number of dimensions self.latent_dim), or a callback function expecting n_dim, n_samples parameters and drawing from user-defined distribution.