Unlock the aspect ratio

This meets the request in [#2](https://github.com/rsantamaria/papercrop/issues/2)

Papercrop was exactly what I was looking for except for the aspect lock.  This changes behavior slightly so that the aspect ratio specified in the model becomes the initial aspect ratio when the unlock option is passed to the helper.  I kept it locked by default so that anyone updating their gem wouldn't need to go fix code.  Simple enough to flip around if needed.