[api] Added RequestDispatcherOptions

[Bertrand Delacretaz <bd...@apache.org>]

I have added a RequestDispatcherOptions class (really just a Map, with
some constants for well-known options names) to these methods:

1) in SlingHttpServletRequest:

RequestDispatcher getRequestDispatcher(Resource resource,
RequestDispatcherOptions options)

2) in SlingScriptHelper

void include(String path,RequestDispatcherOptions options)

Here's from the javadocs of RequestDispatcherOptions:

 *  Typical use cases include:
 *  <ul>
 *      <li>
 *          Forcing a resource type, to render a Resource in a specific way,
 *          like for example <em>render myself in a suitable way for a
navigation box</em>.
 *      </li>
 *      <li>
 *          Adding selectors when including a Resource, like for example <em>add
 *          a "teaser" selector to the request that I'm including here</em>.
 *      </li>
 *  </ul>

Comments are welcome,
-Bertrand