Adding the capability to include MediaRouteButton as an ActionViewTarget

Hi.

In Google Cast design documentation (https://developers.google.com/cast/docs/design_checklist#prompting), it is recommended to provide a Cling type view to introduce the Cast button. This pull request adds the capability to your library to recognize the Cast button (MediaRouteButton) as an ActionViewTarget; a new Type is added and a simple usage would be:

        new ShowcaseView.Builder(this)
                .setTarget(new ActionViewTarget(this, ActionViewTarget.Type.MEDIA_ROUTE_BUTTON))
                .setContentTitle("Cast Title")
                .setContentText("Cast Text")
                .build();

If this can be added to the main library, it makes it easier for developers to use the library directly (preferred approach) rather than using my fork.

Many tanks
Ali.