Added pre-draw event for tiles to allow applications to alter the image just prior to its rendering. This should enable invert, brightness, contrast, sharpen and other transformations to occur naturally through the 'tile-drawing' event listener.

The listener is given the 'rendered' object which is the pre-drawn image on a canvas context object. By altering the 'rendered' object the listener can alter the display results. Since this event gets fired multiple times for the tile, it is wise for the handler to track what their desired modifications are and to tag the 'tile' element to keep track of the modifications already applied.
Added test case for tile-drawing event.
Added message to clarify that the event only fires when <canvas> is being used.
Reworded the test messages to 'should be' form as per standard practice.
Merge pull request #348 from fs-webdev/tile-drawing-event

Added pre-draw event for tiles to allow applications to alter the image ...