Animation support

[tptee]

See the README diff for how the animation API works!

This PR depended on the new constraint removal support in our fork of Autolayout.js.

/cc @boygirl @eastridge @kenwheeler @divmain et al

[coveralls]

Coverage increased (+1.9%) to 86.348% when pulling a232027 on feature/animation-support into 301c1b7 on master.

[kenwheeler]

This looks awesome. On local, when running builder run dev, it sometimes does the intial layout, but resizing the window isn't keeping the positions synced. This is on a fresh pull.

[tptee]

Eww. I'll poke at it tonight!

[tptee]

Repro'd it here. What the hell is going on there? I'll try to track down the regression

[tptee]

I've confirmed that this craziness only happens against React 15. But...why??

[tptee]

This breaks React 15 rendering:

<script>
  // Sanity-check the component loaded...
  setTimeout(function () {
    var content = document.querySelector("#content");
    content.innerHTML = content.innerHTML ||
      "If you can see this, something is broken (or JS is not enabled)!";
  }, 500);
</script>

It's in the demo html files of many of our OSS projects. We should probably check that this doesn't happen elsewhere. @boygirl @kenwheeler @coopy

[coveralls]

Coverage decreased (-0.6%) to 83.849% when pulling 2184a24 on feature/animation-support into 301c1b7 on master.

[tptee]

Going to merge, post-review comments appreciated!