Image Markup: function to increment shape size
Image Markup: add incrementSize method to shapes

Rectangles and Circles get a incrementSize method that increases their
size and keeps them centered on the same point.
Move keyboard key parsing logic to caller

The incrementSize function now takes a keyMap variable instead
of a keycode.
Put in some missing semicolons
Remove hard-coded values from Markup Builder

Instead set those defaults in the shape's class definition.
Add clone functionality to Circles and Rectangles

Add fromObject and toObject functions to these shapes so that they can
be "clone"d
Add custom center function to Rectangle object
Add comments
Get rid of unnecessary complexity in incrementSize

Caller will just pass a Shape and an integer
Merge image-markup--add-resize-keyboard-shortcut (#24) into ifixit-production

Image Markup: add incrementSize method to shapes

Rectangles and Circles get a incrementSize method that increases their
size and keeps them centered on the same point.