Readme typo fixes

@cressie176: This looks like an excellent library, I'm looking forward to using it! A few typos jumped out as I was reading the readme, hope you don't mind that I took the liberty of correcting them.

The spelling of "minimise" didn't look right to me, but I checked and it is the UK spelling (I'm not super-familiar with UK english); the US spelling is "minimize". I suppose it's possible that some of the things I corrected were actually correct UK spellings or idioms, I don't know for sure.

Again, thanks for all your work on this and for contributing it to the open-source community. This is exactly what I was looking for: true BDD (like cucumber.js) that ties into existing mocha tests & test runners.