Add in pyfaust and matfaust.FaustFactory.rand() the possibility to set the...

Add in pyfaust and matfaust.FaustFactory.rand() the possibility to set the density per row or per column of each factor.

- By the way, authorise 'dense'/'sparse' and 'real'/'complex' to be indifferently set as the fourth or fifth argument.
- It also addresses the default density of 5 non-zero elements per row/column asked by #43.