[Runtime] Add application extension skeleton and getManifest, getMainDocument API support

In this patch two API support is added, one is xwalk.application.getManifest(see
http://developer.chrome.com/apps/runtime.html#method-getManifest), the other is
xwalk.application.getMainDocument(see
http://developer.chrome.com/apps/runtime.html#method-getBackgroundPage).

We use asynchronous calls for both of these two APIs. getManifest() will
retrieve current running application's manifest dictionary value and post the
value to renderer. getMainDocument() utilizes ApplicationNativeModule to get
RenderView from the main document's routing ID, then return its main frame
global JS object.