Refactor configuration (and everything else)

Because it stores its configuration in a singleton, the Namely gem can't be used to connect to more than one account simultaneously. This branch fixes that by making a bunch of *non-backwards-compatible* changes.

* Remove the singleton configuration block; instead, use a `Namely::Connection`
* Replace `RestfulModel` with `Collection` (no more class methods!)
* Every instance of a resource is just a `Model`—no more subclassing from `RestfulModel`
* `Authenticator#current_user` returns a `Model`
* Profiles can no longer be updated: this is due to a bug in the API, and should be fixed shortly
* Update documentation
We would, if we had a `resource_name` that didn't match the `endpoint.split("/").last` pattern.

Since we don't, though, there's no sense in keeping it. It would be easy to add it back later if necessary.