Remove dependency of the core on javax.servlet

This is a breaking change but rather with limited impact:
 * `Context.getHttpServletRequest` / `Response` methods were `@Deprecated` already
 * `FlashCookie` / `SessionCookie`'s `save` method now take `Result` as a second parameter, but users are not supposed to call those methods (or are they?)
 * several classes relocated (`ninja.async` → `ninja.servlet.async`, `ninja.utils.CookieHelper` → `cookie.servlet.CookieHelper`, `ninja.utils.ResponseStreamsServlet` → `ninja.servlet.ResponseStreamsServlet`)
 * **biggest breaking change:** introduce a new `ninja-servlet` module where all servlet-related code lives (so that `ninja-core` doesn't depend on `guice-servlet` or `javax.servlet`)

This paves the way for supporting other containers, such as Netty or Grizzly.

TODO: move the test lib to an in-process model, possibly splitting the jetty-based implementation into its own module (or removing it entirely); similarly to how Jersey 2.0 provides several backends for tests.