Add onMouseleave event

[FabienCharlet]

First, thanks for your work !

I want to report a problem when you start to draw a signature in the canvas and release the mouse outside the canvas.
The line is not saved.

Simply add ng-mouseleave="onMouseup()" in the canvas declaration to fix the problem :)

[adrogon]

Thank you for your idea, implemented it under #66.
Also checks whether the left button is depressed, so that onMouseup() is not executed by simply mouseovering the canvas.