retain aspect ratio when resizing and updating layout.
###### Problem story:

If I have an element that needs to remain 100% of my viewports width but yet also retain its original aspect ratio before being resized - how would one go about achieving that?
![image](https://cloud.githubusercontent.com/assets/1205730/11154996/81eb0aa2-89f7-11e5-914c-196e258264a3.png)
###### Proposed solution:

If you lock layerA's width and height and you set a constraint saying layerA = 100% width of canvas it should resize but retain its aspect ratio. 
![image](https://cloud.githubusercontent.com/assets/1205730/11155115/6cb5fe70-89f8-11e5-8104-2093a0e72dea.png)

Currently if you lock a layers dimensions it doesn't resize at all when you update layout.
![image](https://cloud.githubusercontent.com/assets/1205730/11155213/1645b764-89f9-11e5-925f-9a925f1a5ab6.png)

Not sure if my proposed solution conflicts with anything but if it doesn't this would be a really rad addition. 

Non the less thanks for making this plugin, its one of my favorites :) 
