CYB-50 Adding API infrastructure.  Enscapsulates all calls to the Canvas...

... API in this new class

  * Uses Faraday & Net::HTTP::Persistent (keeps connections open, to save setup/teardown)
  * Requires setup in config/secrets.yml with API keys from Canvas (see README.md)
  * Some specs do actual calls, so for them to run it requires an instance of Canvas.
  * Some rewrite on README.md
This string is the User-Agent that our Datacultures APP will use when querying the Canvas API.