Simple XmlHttpRequest object
Its interface might not be obvious enough for junior programmers in a hurry. I suppose there's some assistance in your README. Anyway the code is quite clear given a little experience or effort. It doesn't use exactly the subset of JavaScript that Douglas Crockford recommended in JavaScript: The Good Parts, but that doesn't always matter.

I like this code. It has enough complexity for the real world without taking on responsibilities extraneous to wrapping XmlHttpRequest (or its equivalents). Nice use of closures. I really like that you incorporated a hyperlink to a line of github code that you found helpful.