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. 
"test" prefix is redundant with Junit 4+.  Does our coding standard say we should use them?