V5 API upgrade

This upgrades the API usage to use the v5 REST API (as detailed at https://www.pivotaltracker.com/help/api/rest/v5). I haven't added in any of the new bits (e.g. activity data), but I've just ported over the existing bits so that we can now later add in the new bits.

Note that
* XML stuff has been replaced with JSON throughout, as the new API doesn't seem to support XML any more
* Authentication via username/password has gone away, in favour of API tokens which are only findable via the Pivotal profile.