SWC-1496: World Clock Tools

This is my start so far.  This isn't really ready for merge-in, but I wanted to check in that the architecture I'm moving towards is appropriate and makes sense.

Basically, I'm creating a GWT RPC service to handle the needed time related requests, and I've added a simple data class for transporting the needed information between server and client (`DateTime`).
Is this a problem, or do we just need to comply with the license?  Because we can also use joda-time; I picked threeten because joda's API isn't as nice and this has a clear upgrade path for if we move to java 8.