Implementation of astropy.vo.samp from SAMPy

Since @lpaioro no longer has time to port SAMPy to ``astropy.vo.samp``, I have rebased his branch (which also includes many contributions from @cdeil) and re-opened a pull request. This is still work in progress. Here is the to-do list copied from #972 which is now superceded:

- [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