sandbox http resource needs to support multipart post

We need to be able to do multipart POST/PUT requests.
It'd be worth our while to just decide to use [treq](https://github.com/dreid/treq/) for this stuff in the resource. It has nice things for [multipart](https://github.com/dreid/treq/blob/master/treq/multipart.py#L17) and dealing with [timeouts](http://treq.readthedocs.org/en/latest/api.html#treq.request).