Remote shutdown via admin command

Hi,

This adds support for the feature requested in #52 - you can post an empty request to `/__admin/shutdown` to cause WireMock to stop itself.

I'm not _super_ happy with WireMockApp having a reference to WireMockServer (which is why I created a minimal interface, TerminableServer), but I'm not sure what a better alternative would be - suggestions welcome, of course.

I also considered updating JUnit to 4.11 so I could use `expectedException.expectCause()` in the test, rather than write my own matcher, but I think you're intentionally still on JUnit 4.10 so you can test compatibility?

Thanks,
Rowan