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

ActiveSupport#as_json turns out to be really expensive, so calling it
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. This improves performance by more than 200%.

If however the presenter is passed to the serializer directly, it does
have an as_json implementation that will do the right thing.

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.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>