64-bit and architecture-agnostic platform support

Closes #1.

@danthewolfe, thanks again. It would be cool if you could have a quick look at this.

Is the [readme](https://github.com/adam-lynch/platform-overrides/blob/64-bit/README.md) clear enough? Am I missing a test? You can see the names of the tests here: https://travis-ci.org/adam-lynch/platform-overrides/jobs/42582537#L81