Add warning to InMemory Service Registry

Partially addresses: https://github.com/Jasig/cas/issues/548

This pull adds a warning to the InMemory Service Registry to note/stress the InMemory nature of the persistence backend. This is to allow CAS adopters to better understand that changed made at runtime will be lost upon container restarts. 

Also added some test cases for the class. 
That's a fine idea. I have got some CS and FindBugs items planned ahead. I'll account for this as well in the standards. Would be a lot of noise to get this fixed as you noted, we have a mix right now but consistency one way or another would be even better.