Add `trunc`, with a polyfill for browsers that don't support it.

[rgrempel]

Of course, you can implement this in plain-old-purescript, but it's likely that the native browser method is faster, when available. And I've included a polyfill for cases where it's not available (mainly IE, according to this page)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/trunc

[rgrempel]

I re-pushed the commit with a small whitespace change -- jscs was otherwise complaining about whitespace.

[garyb]

Thanks!