Permit scanning of environment

[thomas-p-wilson]

It would be immensely useful to have dropwizard-swagger permit scanning of the Dropwizard environment for resources, rather than an entire package. My primary use case is that I have a resource I am working on, but have not yet registered it. Should I choose to deploy the application, that resource will be documented by Swagger as it resides in the package I have told it to scan. I propose allowing Swagger to scan the Dropwizard Environment in the event that the resourcePackage is null or empty.

I have a forthcoming PR for this.