Better documentation of mcmc hyperparameter

[ibayer]

Access to the mcmc hyperparameter is fairly obscure and needs better documentation.

• add documentation to python mcmc module
• add function that generates string parameter description
• add documentation to C fastFM-core/src/ffm.c L. 77 ff

Hyperparameter layout in fm.hyper_param_:

    print (['alpha'] +               
    ['lambda_w'] + ['lambda_V' + str(i) for i in range(rank)] + 
    ['mu_w'] + ['mu_V' + str(i) for i in range(rank)])`

output example (rank=4):

['alpha', 'lambda_w', 'lambda_V0', 'lambda_V1', 'lambda_V2', 'lambda_V3', 'mu_w', 'mu_V0', 'mu_V1', 'mu_V2', 'mu_V3']