Add new function FaustFactory.eye() and modify/add/fix several things about...

Add new function FaustFactory.eye() and modify/add/fix several things about concatenation, addition, substraction and real-complex conversion of Fausts.

- New function FaustFactory.eye().
- Impl. add/sub of a Faust with a matrix.
- Fixing issue #72.
- Adding missing unit tests for pyfaust.Faust.__add__()/__sub__(), and test add/sub of real Fausts with complex scalars and Fausts(for pyfaust and matfaust).
- Converting real Faust to complex Faust when concatenated/added/substracted with a complex Faust.