Add introspection features

Currently there's no way to find out what you can do with a launchpadlib resource, apart from looking at the reference docs or the wadl. It would be very cost-effective to implement dir() on Resource. It would probably be a wrapper around a wadllib dir(), which would look at the WADL representation of the resource and return the list of followable links.

A little less cost-effective but still useful would be lp_fields, lp_links, and lp_collection_links methods that tell you which fields are which.