Consume JSON-LD metadata from a paged API

[nein09]

• Add a new api source type, that takes an URL template, and pages through it
• Add tests for the api source type
• Add a little documentation for the new sourcetype

You can test it out on CCADI's endpoint but it'd be even cooler if you had a different paged API to try it out on:

- sourcetype: api
  name: ccadi
  url: "http://hedeby.uwaterloo.ca/api/metadata?page=%d"
  properName: ccadi
  domain: http://hedeby.uwaterloo.ca
  apipagelimit: 200

Other things I did while I was in here:

• Move some core acquire functionality into functions so that I can call them from the api sourcetype code
• Set application/ld+json as a constant in the summoner module

[valentinedwv]

Looks good. New function for getting JsonLD should be useful, and testable