Adds abort_custom() wrapper.

This lets us return error with 200 codes when the api is being called with a JSONP callback.
JSONP only works if it actually gets something back, so returning 404/403/etc breaks it.
Cheat on REST a little by including an HTTP_status_code field in /item response

This way clients that can't read the real HTTP response (like those using JSONP)
can still have access to the RESTy goodness.
Merge branch 'master' of github.com:total-impact/total-impact-core