Add "metadata" as a possible parameter for invoke request

[kwallis]

https://developer.blackberry.com/html5/apis/blackberry.invoke.html#.invoke

The request object can have another "metadata" parameter.
We should add an example of how to use, perhaps the facebook invoke example:

{ action :"bb.action.OPEN", target: "com.rim.bb.app.facebook", metadata: JSON.stringify({object_type : 'page' , object_id : '328506290597521'}) }

From Cody:

The metadata attribute is a publicly available attribute in our APIs. The attribute is used in much the same way that “data” is currently, however the whole idea around metadata is that it’s more ‘optional’ for usage. Ie. the content there is mostly supplementary to the content that is provided in either the URI or data attributes. So each client can utilize it slightly different and pass something slightly different, it’s up to targets to understand it and utilize it (if they so choose).

[kwallis]

cc @jeffheifetz @bryanhiggins