Move comments from samp core.py to __init__.py
Move extensive example to sphinx docs; minor cleanup
Remove unused includes
Rename astropy.vo.samp.core to astropy.vo.samp.utils
Break up astropy.vo.samp into multiple files.

This was discussed and agreed on in GH #972.
In addition to the structure suggested there by @lpaioro,
I put the constants and errors in separate files.