Fixed aspect ratio while editing L.Rectangle

I need a way to draw rectangles with fixed aspect ratios in Leaflet. This is my first throw at it using Leaflet.draw, please let me know if you are interested in the pull and if I need to change some things.

I'm especially not sure about the way to set the aspect ratio, and if it should be set on the `Rectangle` or on the `L.Edit.Rectangle` class.

I did not yet look into creating new rectangles with a fixed aspect ratio...