Browser and AMD module-loader support

Hi, I needed to get Slugg working in-browser (and via an AMD loader) which lead to me forking/creating this branch.

If this isn't something you want then feel free to close, I can just maintain my fork and publish seperately (assuming that's OK with you).

Otherwise, if you want this stuff but disagree with any of my choices (I've used a couple of my own modules to get the tests to run in-browser) then let me know, I'll be more than happy to make tweaks.

Thanks for building this, by the way – it saved me some time! :)