smil

Use SMIL for all MMS attachments.

Also, increase MMS maximum size to a bit under 300kb (chose 280 for some padding), as well as allow images to be their recommended max dimension of 1280.

As is pretty obvious by the files changed, most of the changes are copying in the W3C standard SMIL DOM implementation with some modifications for simplicity. Since TextSecure doesn't attach multiple images in one MMS, I was able to simplify the logic found in other messengers much more drastically which is nice. The actual code that assembles the SMIL document can be found in SmilUtil.