Add .include_root_in_json, .defaults, disable root key by default

Since ActiveRecord 4.0, root keys are disabled by default. By adding
a similar option and setting it to fall back to the ActiveRecord
setting, we get a nice expected behavior out of the box, and the new
.defaults method allows for fine-grained configuration if desired.

Readme updates coming soon.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Signed-off-by: Stephen Celis <stephen@stephencelis.com>