Shutdown hook

This is almost exclusively Greg's work.

I tested it and reintroduced a unit test by adding more mocking.

I looked at ModeShape and the calls we're making to ModeShapeEngine appear to be the same that the JcrRepositoryFactory made.

I have pull requests for fcrepo-message-receiver, fcrepo-module-auth-rbacl and fcrepo-module-auth-xacml that simply change the spring configurations in the way that has been done in this project.

See PR #260 for earlier commentary or the ticket in pivotal.
https://www.pivotaltracker.com/story/show/66454096
It really is "Exception". See: 
```
JcrRepository.getStartupProblems() throws Exception
```