Joker-broker

* AbstractModel.find: update_cache() and mark_permanet()
* gen_random_string() now uses os.urandom, and is unexposed

* AbstractModel._update_cache -> AbstractModel.update_cache
* remove AbstractModel.load_with
* new AbstractModel.find, returning objects in stead of pk

* move MultiFernetWrapper away to joker.masquerade

* add SQLInterface.execute
* remove redundant Table cache from SQLInterface, which is done by SQLAlchemy already
* remove client_encoding for sqlite
* start to write README
* setup_userdir, get_resource_broker. No magic at broker.__init__
* less redundancy creating a new property interface

* add methods `find` and `load_with` to `AbstractModel`

* user directory (`~/.joker`) and default config file creation
* bugfix: StaticInterface.get lacks *args so that `si.get('DEBUG', False)` fails
* joker.broker.interfaces.sql -> ...sequel


[1] http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#when-do-i-construct-a-session-when-do-i-commit-it-and-when-do-i-close-it