Crawl the API à la HATEOAS

[tripu]

Done:

• Crawl the API in a HATEOAS manner; ie discovering entities, properties and links dynamically instead of hard-coding them. Fixes #15.
  (This does not ensure that all data from the API is retrievable now; we still have to teach Apiary how to map certain patterns to HTML elements. Also, there are some inconsistencies in the API that have to be resolved for certain properties to work in Apiary — see “to do” below)
• Always invoke the API with embed=true, to save on HTTP requests sometimes. Fixes #18.
• Implement a very simple cache, that ensures no API URL will be called twice.

To do:

• Update README (will do in a separate PR).
• Update the documentation comments of a few functions that have changed here.
• [x] w3c/w3c-api#32.
• [ ] w3c/w3c-api#29.
• [ ] w3c/w3c-api#30.

[Edited 17/Sep to reflect that this depends on w3c/w3c-api#32, not really w3c/w3c-api#28.]

[tagawa]

Works for me, with the exception of team_contacts which you mentioned.