App View: CursorResponder interface.

Added methods as discussed in the meeting to the CursorResponder
interface.
App View: Stubbed Node methods

Stubbed the methods necessary to implement App View.
App View: Stubbed Wire Methods

Stubbed the methods in Wire necessary to implement the Window class.
App View: Window class

Implemented CursorResponder methods for Window.

Also added the following methods:

 * activate:    Activates the context as the current
 * view:        Accessor method for the active layer
 * add-subview: Add a view to the active layer
 * _snap-item:  (private) Finds the nearest item to snap to
                Has the added convention that "snappable" items
                are *selected* (in paper.js)
 * _find-item:  (private) Finds the nearest item (not selected only)
App View: Removed view method

The View method was only being used in one place, so I have removed it
(Also, the Window class is itself a view class, so it doesn't make
sense for it to have its own view property.)
App View: Made Window::activate a void function
App View: Documentation for window.ls
App View: Re-ordering initialisation in App

LeapCursor should be initialised after Window because Window sets up
the current drawing context.
App View: Ensuring Paper resizes viewport on canvas resize.

Without this flag, the aspect ratio of Paper's viewport is messed up
by resizing the screen.
App View: BUG FIX: App requires a DOM element.

$(\#window) returns a jQuery object, whereas we need an
HTMLCanvasElement
App View: Changing add-subview to insert-children.

In keeping with the paperjs API.
App View: `select-at` sending paperjs doubleclick callback now

...instead of the `selected` method.

This allows for interoperability with mouse events, and additionally
means that standard paperjs Items can be used.
App View: Further Documentation

Documented the following:

 * app.ls
 * window.ls : Constructor
 * cursor.ls
App View: Fixing Typo on Wire.

Wire::set-end should take a point and update the end (input side)
of the wire.

Wire::set-begin does the same for the beginning (output side)