BatchSender: using CORS POSTs to send the data

**PLEASE DON'T MERGE**

This gets rid of the img GETs in favor of CORS POSTs. BatchSender is way simpler now, woot!

This still needs to be tested more, TRUST being down this weekend is preventing that for now. But want to get this out there for feedback.
XDomainRequests don't allow you to set headers, so there is a risk that IE8 can't send JSON with CORS. If that is the, case we'll have to switch the beacon to accept form data instead.