Added Java config support to Message Servlet

Added Java configuration support to `MessageDispatcherServlet` so that it can be configured within a `ServletContextListener` or a `ServletContainerInitializer`.

The change was extremely minor. Biggest change is the Javadoc. I added a new constructor to call `FrameworkServlet#FrameworkServlet(WebApplicationContext)` and then copied the JavaDoc from `FrameworkServlet` to the new constructor (exactly like it was done in `DispatcherServlet`). Works great!

What's the roadmap for 2.1.4? I'd love to see this committed and 2.1.4 rolled out soon.

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Issue: SWS-832