Add in core cpp lib the capabililty to write a FAuST into a Matlab file and link the Py wrapper to it.

- This is only for CPU Faust Transform.
- It was an already supported functionality of wrappers. Now it's in the core lib (useful for possible next wrapper).
- It has been tested OK for py2 and py3.
- The Matlab wrapper is not yet linked.

Code added or modified:
- Faust::Transform::save_mat_file() into CPU/faust_Transform.h(pp).
- Faust::MatGeneric::toMatIOVar() (abstract function) and definitions for MatDense and MatSparse.
- FaustPy.save(): add Matlab_core as a possible value to format argument (it's the way to use the save_mat_file() from core lib.).
- quickstart.py: add a test snippet for save().
- setup.py.in: add matio as a dependency library (used for compiling faust core with cython).
- add save_mat_file() cython binding in FaustCoreCpp.h(pp), FaustCoreCy.pxd, FaustCorePy.pyx.