Generalize config to connect to multiple accounts

* Add Connection, Collection, and Model classes
* Implement #index, #find, #exists?, and #build on Collection
* Persist Models with #save
Reimplement collections

* countries
* currency_types
* events
* fields
* job_tiers
* reports
Update documentation to reflect the new interface

* Add YARD for Connection, Collection, and Model
* Update examples in the README
Remove the configuration singleton

* Danger: breaking changes!
* Remove configuration block
* Remove RestfulModel and its subclasses
* Authenticator#current_user returns a Model, not a Profile
Remove old RestfulModel fixtures

* Use @example.com for example email address
* Remove resource_name instance variable from ResourceGateway
Integrate Connection and Collection tests