Use hardwareConcurrency when available

Use `navigator.hardwareConcurrency` when its available for `util.estimateCores`. Also fixed a few missing inverted commas around the typeof checks. Closes #147
Tidy up style

Changed the comparison with undefined to a `in` check. Sorta matches the rest of the code better :smiley:
Merge pull request #158 from jduncanator/concurrency

Use hardwareConcurrency when available.