Support for CORS requests

Biodalliance can stream data over HTTP if the server supports CORS.

To see this in action, head over to [biodalliance](http://www.biodalliance.org/human37.html), click "+" and add `http://hammerlab-dev3.hpc.mssm.edu:9876/datasets/dream/data/training/normal.chr20.withMDTags.bam`.

See [this article](https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS) for details on CORS.