Finish promisification refactor

[malept]

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Summarize your changes:

With this PR, all of the codebase has been converted to use promises instead of callbacks. Additionally, the code has been largely restructured to use ES6 classes, so it's hopefully easier to follow. This is obviously a really large PR that isn't easy to review (and it was difficult to find any place to split out work), so it may be easier to look at the new code and 🤞 🤞 the tests all pass (they all passed locally and Travis passed when I ran it on the branch).

run-series has been dropped, but callback support is retained with nodeify. I don't see dropping support for callbacks in packager(...) any time soon.

The breaking change is that when you specify multiple targets, they are run "in parallel", due to the usage of Promise.all. This fixes #123 to some extent.