(#147) - Implements logging & /_log 

An as-exact-as-I-can-make-it clone of CouchDB's /_log. Because. Based on #144
@nolanlawson This particular case just logs:
Successful cookie auth as: "marten"

The place where HTTP requests are logged would indeed also log the Authentication header (if the debug level says so). CouchDB does that as well, it's one of the reasons why /_log is admin accessable only. The other is that it's a way to discover document names that you otherwise might not guess (e.g. because they use a uuid as _id).