Allows SystemJS to load within Web Workers

This adds support for web workers building on the work done in ES6
Module Loader. Mostly just window checks, did have to do a plain old
eval in this case. Fixes #165
Fixed node tests with new WebWorker support

Previous changes to make SystemJS support WebWorkers mistakenly broke
Node, this fixes it.
Merge branch 'worker' of github.com:matthewp/systemjs