jersey: Make exception mappers configurable

This PR adds a parameter to server configuration called **registerDefaultExceptionMappers** that defaults to **true**.  If set to **false**, the dropwizard-provided ExceptionMapper classes are **not** added to Jersey.  

This is required if you want to customize exception handling since Jersey does not allow you to unregister classes that have already been registered.
Doesn't this change the behavior of tests? With the current implementation it would add all those exception mappers in test runs which hadn't be there before.

Maybe I'm overlooking something?