(#109) - Adds /_security support.

This completes the work started in #103 and should be enough to close pouchdb/pouchdb-server#32. (Also 'completes' #107, which was already marked fixed).

If there are any authorisation problems left to be found, from this PR onward they're considered bugs by me.

This PR contains a lot of ``{req: req}`` options objects, if there's a way to get the 'current' request object without passing it in all the time, that would make the code a lot easier to maintain. I've seen some solutions but I doubt they can follow levelup callbacks, without (too much) extra code. Suggestions welcome, although it's not a show stopper.