Use Agg backend when saving images. Add option for UTC times.

[davebx]

I did some basic testing of this in OSX, and it seemed to work correctly.

[nekrut]

@arq5x is there anything we can do to help push this through. We are trying to get this going in Galaxy. Tx.

[stevenweaver]

The check seems to be failing due to an error in the CI build configuration itself.