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. 
Not so much a JUnit4 thing here; it reads easier. i.e. "Test the Save Method"