First `SAMPy` code inclusion within `astropy.vo.samp` package

This is a preliminary work addressed to add IVOA SAMP messaging capabilities to `Astropy` package, through the inclusion (and adaptation) of SAMPy module (see https://pypi.python.org/pypi/sampy/). It includes a Python package derived from `sampy.py` module, and a script for the SAMP Hub execution.

Presently the code is rather close to the original SAMPy code, therefore it needs some work on it. Here the TODO list:

- [x] Change the license
- [ ] Change the coding style
- [x] Make it compatible with Python 3
- [ ] Rewrite the documentation
- [ ] Make it fully compatible with the protocol v.1.3
- [x] Write tests

Many thanks to Thomas Robitaille and Christoph Deil for their help. Any other contribution is greatly appreciated.