BREAKING CHANGES gh-1014

This fixes #1014 but introduces two potentially breaking changes:

* Calling `ractive.animate()` will cause the first changes to happen immediately, rather than on the next `requestAnimationFrame` frame. For interpolated values this changes nothing, but for values that can't be interpolated (i.e. strings), it means the view updates synchronously to reflect the final value
* The `cssLength` interpolator has been removed. It was never documented or tested, was the cause of #1014, and is unnecessary - so good riddance.