Dependency (py)llist

[jayvdb]

I suggest making pyllist a mandatory dependency in setup.py, or at least documenting how stash.py can be used without either of pyllist/llist, as currently the default usage in the docs fails because pyllist and llist are not pre-installed.

Then an extras_require can list llist as an optional dependency, to allow people to see that llist is supported and documentation can explain that it provides a performance boost.

Also jsonpickle, msgpack and apsw dependencies should be listed in extras_require. fwiw, it would be nice to document the benefits of apsw over the sqlite3 module.