Performance: add benchmark spec, Jsonite#present method, skip #as_json by default

`ActiveSupport#as_json` turns out to be really expensive, so calling it recursively every time we presented an object added up to a significant amount of CPU time. Most often the JSON serializer will call `as_json` itself, so we don't have to. If however the presenter is passed to the serializer directly, it does have an `as_json` implementation that will do the right thing, and only once.

This improves performance by more than 200%. See both commits to see the progression here (reduces time to present 1750 objects from 250ms to less than 100ms).

This also pushes resource collection logic (arrays and relations) into the presenter instance itself, so something like `Presenter.new(ary)` will now actually work as expected.