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.
@joschi This actually does not change test behavior.  There are a few different scenarios, keeping in mind that most tests never touch DropwizardResourceConfig:

** Tests that directly use DropwizardResourceConfig **
A few tests directly use this class, like the auth tests (example: ChainedAuthProviderTest.)  They pass in **true** for **testOnly** so previously were disabling the addition of the exception mappers.  This stays the same, as only code inside the !testOnly block was changed and AbstractServiceFactory is not used (so no exception mappers are added, which was the previous behavior.)

** DropwizardAppRule **
Tests that use DropwizardAppRule did not have a special handling so **testOnly** was false and that code gets executed.  The exception mappers don't get added there, but they DO get added in AbstractServerFactory because the **registerDefaultExceptionMappers** config bool defaults to true.  So, the behavior of exception mappers getting added for these tests stay the same.

The only way the behavior could change is if there are tests using DropwizardResourceConfig with testOnly = false and expecting the exception mappers to be added, but I can't find any tests doing this currently and it would really make sense, since you would only care about that if you are hosting this in Jersey and you probably won't be doing that w/o using AbstractServerFactory or more likely, DropwizardAppRule.