Support for relative values in animation/state properties

So you can do things like:

```coffeescript
layer.animate
    properties:
        x: '+=50'
        opacity: '-=0.1'
```

This also adds support for taking in strings with units as property values, e.g. `50px`. The units will be disregarded since we don't do anything with them at the moment, but we might in the future.

All tests pass and I've added new ones for the relative properties.