Moved session.py to lib/session
Extracted the SessionMiddleware from old session.py to its own file in lib/session/middleware.py
Implementation of converter for values that will saved in the
cookies.

ValueConverter provides only the serialize and unserialize Methods, and also
the encode and decode stubs

PlainValueConverter -> value => encoded_value
SignedValueConverter -> value => {"algo": "sha256", "value": value, "sign": <signature>}
CryptValueConverter -> value => {"algo": "AES", "value": <crypted value>}
Collect all converter for more convienance
Added Tests for the Converter.
Added Test for the Session Class,
needs more improvement.
Added option to select session implementation in adhocracy.ini.in
`adhocracy.session_implementation = beaker` as default

Added overwrite for `adhocracy.session_implementation = adhocracy_session` to hhu buildout

Made middleware available of the new option.