Fix scaling

Scales template images to fit in the cell bounds. Creates the mask context at the device's scale, so it will work properly on retina displays with `@1x` or `@2x` images.

![screenshot 2014-04-21 09 43 12](https://cloud.githubusercontent.com/assets/594059/2755643/d65126c4-c96b-11e3-9079-1088e11c1376.png)

Fixes #1 and #2.