adminPort isn't used with dynamic http.port and http.adminPort

If `http.adminPort` and `http.port` are both set to `0`, the `internal` servlet is added to the `main` connector at context path `/admin` and the `internal` connector is unused.

[ServerFactory.java#L317](https://github.com/codahale/dropwizard/blob/v0.6.1/dropwizard-core/src/main/java/com/yammer/dropwizard/config/ServerFactory.java#L317) should be changed to match [ServerFactory.java#L100](https://github.com/codahale/dropwizard/blob/v0.6.1/dropwizard-core/src/main/java/com/yammer/dropwizard/config/ServerFactory.java#L100). Maybe adding something like `HttpConfiguration#hasSeparateAdminPort` would be helpful?