Created an object which can be a central registry for keyboard

shortcuts on the current page. This new object keyBindings is 

owned by RB.Pageview. Subclasses of RB.PageView (e.g., diffViewerPageView 

and dataGridPageView can extend this new object to have their own

keyboard shortcuts.
Each key set can map to two things: callback mapping to a callback

function and help (temporary mapping to a str text, can be a foundation 

of future features).

The old keyboard shortcuts on diffViewerPageView are still working.
Tested on Chrome and Firefox.
Tested in dataGridPageView by adding this dummy code segment:
...

keyBindings: _.extend({
    'a': {
        callback: 'testAlert',
    }
}, RB.PageView.prototype.keyBindings),

testAlert() {
    alert("This is a test alert.");
},
...