Add :with option to Jsonite.property

This allows us to embed resources directly (if we don't want to comply
with HAL's _embedded requirement).

I'm also removing the type-checking. If a class accepts an object and
responds to as_json (a common presenter pattern), it should work.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Add spec for throwing :ignore

Throwing :ignore removes the node from the final JSON.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Update README

Includes HAL resource embedding.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>