JQuery dependency and backbone view

[SylvainCorlay]

I would like to eventually allow users to write widgets using backbone native views or d3 views or other drop-in replacements for Backbone.View using the DOM manipulation library of their choice.

These replacements for backbone views share the main View.el attribute, which is a native DOM element. Hence it should be used to insert the widget view in the page generically rather than View.$el which is specific to jquery.

Besides inserting the widget in the page, jquery is only used for some basic css styling and $.proxy.

In this PR, I Widget.js I removed the jquery dependency in manager.js, and Widget.js.

In the very end, I do:

var WidgetView = Backbone.View.extend(WidgetInterface);
var DOMWidgetView = WidgetView.extend(DOMWidgetInterface);

which I would like to move to a different file. The idea, is that anyone could define their own version of DOMWidgetView based on NativeView:

var NativeWidgetView = Backbone.NativeView.extend(WidgetInterface);
var NativeDOMWidgetView = NativeWidgetView.extend(DOMWidgetInterface);

[SylvainCorlay]

@jdfreder will reopen after the widget split.