First cut at minimal API docs package

@davegolland API docs, including an untested handler to serve them, and a fn to write them out to HTML.  I tested the latter on one of the test namespaces (modified to include some defschemas) and the output looked reasonable.  I think this can be included in guesthouse by letting the handlers, calling api-docs on the infos, then consing it on before applying routing, but haven't tested this.  Could probably also use some love in docs and cleanup, but as long as it works probably suffices for a first release.