Default cometName to Empty in findOrCreateComet[T]
Don't call lift.registerComets if there are none.

Before we were sending down the lift.registerComets
invocation with an empty object even in cases where there
were no comets to send down with an AJAX response; we
now generate no additional JS in those cases.
Return XHR objects from ajaxGet.

jQuery returns an API-compatible object. We do this so that
we can properly abort requests when comets need to be
reset.
Don't invoke AJAX failure callback on abort.

We only abort to reset comet requests, and in those cases
we explicitly handle retries as successes rather than
failures.