Make it work in a Web Worker too.

One of the few properties in common between browsers and a worker
environment is the `navigator` property. `window` is also a no-go.
Buster update to v0.7.8.

See <http://docs.busterjs.org/en/latest/changelog/#v0-7-4> for a summary of
breaking changes.
Merge pull request #3 from espadrine/master

Make it work in a Web Worker.