Add application extension and getManifest(), getMainDocument() API support

The application extension is an internal extension to handle application level
APIs(e.g., runtime events, manifest, package management, etc.).

In this commit two APIs 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() retrieves
current running application's manifest value from browser process.
getMainDocument() first fetch the main document's routing ID from browser
process, then utilizes ApplicationNativeModule in render side to get RenderView
of it and return its main frame global JS object.
Add application API browser test for getManifest and getMainDocument