Standardize tracker object / behavior

[bcongdon]

Trackers should be objects, instead of formless modules. They behave more like objects already, and it's more idiomatic to call duck-typed functions of a subclass to Tracker than to a module.

Trackers should be able to do the following:

• Authenticate the API client
  • handleAuthentication()
• Do some type of regular scraping function
  • scrape()
• Do a complete 'refresh' of their object's database
  • backfill()

Trackers should be distinct from their associated models, and should only have simple CRUD tasks on the actual data.

[bcongdon]

Added as of 7d6a252