Making ssp more PyPI-compliant

Hello Phil,

Below is a set of changes that would make the ssp more PyPI-compliant. 
Be aware that I've quickly tested all the scripts, but not all the possible combinations of parameters. Furthermore, I may have introduce some errors in the namespaces (e.g. when changing 'ssp.xxx' into 'core.xxx'  or 'from ssp import *' into 'from . import core', etc.). Feel free to merge if you feel ready and don't notice anything weird. Adding some unit tests at some point would be strongly welcome.
What is on PyPI currently corresponds to the content of the fork bioidiap/ssp. In particular, this implies that ssp might now be installed just by doing a 'pip install ssp' (requires root grants; you might append the "--user" flag otherwise) and adding a dependency to it is straightforward by adding 'ssp' in the setup.py of any python package.

Cheers,
Laurent