implement status resolution w/ caching

[chadwhitacre]

Reticketed from badges/shields#84.

Currently we hit an HTTP backend to resolve a token such as (travis-ci, rails/rails) into (build, passing). This will need to be a well-documented API as it will be how we on-board third-party services.

[chadwhitacre]

@jbowes @nathany This is probably the biggest piece of work before we can switch. Should we talk about what's required here?