Session / secure cookies

Hoboken should expose a secure cookie interface (in hoboken, not in the objects themselves). Should use [itsdangerous](http://packages.python.org/itsdangerous/) to implement this in a safe manner.

Perhaps, we should provide a more generic 'session' interface, and then have one backend be the secure cookies. Other backends should include [Beaker](http://beaker.readthedocs.org/en/latest/) and possibly [dogpile.cache](http://dogpilecache.readthedocs.org/en/latest/usage.html). This should probably be an extension.