Improve how aspect ratio sizing is handled
Currently does not work as well with layers that need aspect ratio sizing (ie. images).

This is currently possible by using Sketch's aspect ratio sizing:

![](https://cloud.githubusercontent.com/assets/1414704/10907971/bb5ffe16-81f3-11e5-9341-99c22c7f36bb.png)

However, when aspect ratio locking is enabled in Sketch, this plugin is only able to change the layer's height. Sketch seems to ignore changes to width.
