Extend pyfaust.Faust.normalize() and add unit tests.

- It's the same than what's been done for matfaust.Faust.normalize() in 84752866 and fe0e2a6f.
- Issue #52 related.
- Some weird bugs in unit tests are left for later (because they are wrong-negative tests).
- It updates pyfaust.demo.bsl to use 2-norm explicitely when normalizing because for pyfaust the default norm is Frobenius (also does minor fix on figure plotting).