[WIP] Pricing Attachment Observer

##### Refactors
- `recurly.pricing.attach` now acts as a factory for the new `Attachment` local object prototype.
- `recurly.pricing.attach` used to intertwine heavily with the `Pricing` prototype. This dependency is now injected via the `Attachment` initializer.

##### New Features
- `Attachment` triggers an observer on the passed element. Changes to the DOM subtree will initialize a dirty check. If the subtree's recurly elements are not as they were before the change, `Attachment` will refresh its bindings.
- This eliminates the integrators' need to understand when the pricing attachment needs to be rebound, and makes for a much more efficient rebinding process.
- The present way of refreshing bindings is rife with opportunity for memory leaks. This controls the rebinding much more closely, reducing the risk of leaks.

##### TODO
- Tests

/cc @cbarton @bhelx 
Not 100% on the naming here, since the factory is also called `attach`. Open to suggestions.