Timeouts for SIA requests.

This morning we ran into a problem where too many SkyView requests were running simultaneously and so we had to reject some image requests. One of the reasons this happens is that SkyView accesses some data from remote services using the Virtual Observatory Simple Image Access (SIA) protocol. If those services are unavailable our requests can hang and build up on the system. Today we implemented a timeout for such requests of 15 seconds. There is also a new option, SIATimeout, that can be set when using the SkyView JAR file.