Pre-release features and bug fixes

* Preserve background color in snapshots (fix #80)
* `LC.util.last()`, `LC.util.sizeToContainer()`, `LC.util.combineCanvases()`
* New `LiterallyCanvas` methods
  * `canUndo()`, `canRedo()`
  * `canvasWithBackground(backgroundImageOrCanvas)` - like `canvasWithExport()` but overlaid on a background
  * Move resize-to-container code into `LiterallyCanvas` so you don't need the jQuery plugin to use it