Add in code to deal with Gaia transients

[martinjohndyer]

As suggested at the telecon, these would be interesting events to have automatically followed up. In particular they would be useful for the pipeline.

They're well located enough to be located in a single tile, and we could do the deeper 5 x 120s exposures.

The only tricky part will be parsing the VOEvents, since they're not formatted in the same way as the other GCNs (we receive them through listening to 4pisky instead of just NASA).