Cesium 2

There is not official plans for Cesium 2 yet, but @mramato and I have been discussing potential breaking changes and this is the place to track them.
- ES6 modules. OL3 example using Rollup, Webpack, and Browserify: https://www.npmjs.com/package/ol
- Clamp to ground (terrain) by default throughout.
- Better modularization overall, perhaps coarse-grained modules that can be hosted on npm. Consider [Lerna](https://lernajs.io/) to manage these in a single repo.
- Consider coarse-grain CPU-intensive subsets in WebAssembly (with asmjs fallback)
- Upgrade to modern promises, CC #3967

We've also started to label some existing issues as marked for 2.0. [Link](https://github.com/AnalyticalGraphicsInc/cesium/issues?q=is%3Aopen+is%3Aissue+label%3A2.0)

No file, test, or entry point is named. Start by reviewing the linked 2.0 issue list and the promise discussion in #3967, then clarify which proposed breaking change is being pursued. The issue provides no concrete acceptance criteria, so completion cannot yet be determined.

Written by the indexing model from the issue text.