Refactor loading to use broccoli-rollup

[ivanvanderbyl]

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We currently employ a rather complicated use of recasting to munge the D3 source into something which is digestible by Ember CLI. This means using a build process which isn't reproducible in the same way as the official D3 builds. To fix this, I propose that we change the internals of this addon to use https://github.com/chadhietala/broccoli-rollup instead, with a similar config as the official D3 builds. This should also reduce build time in projects using this addon.