Added abort function to sync's returned promise, to comply with XMLHttpRequest

I extended the Promise returned by "sync" to also include an "abort" function. This is to comply with "XMLHttpRequest" and more specifically, jqXHR.
This is needed because the original's Backbone sync returns a jqXHR object, which has an abort function, which is used in some use cases, so not having it in iobind causes an error.