Add cacheBust option

[tremby]

This addresses #70.

[MoOx]

I was about to cut a release but got this issue locally:

# test xhr request
# - request is made, gets a result, and is cache-busted
not ok 1 Error: TypeError: 'undefined' is not an object (evaluating 'r.responseURL.indexOf') on line 12906

Any idea if that can be cause by what you did here?

[MoOx]

(I think tests are running via safari on my machine, testling is choosing by itself)

[tremby]

The tests ran OK for me in Firefox. It's possible there's a bug. I'll look into this when I get time (in the next few days), if you don't find the issue before then.

[tremby]

It's working fine in Safari 9.1 but I can reproduce the problem in Safari 8. Looking into it.

[tremby]

I've fixed the test for Safari 8 in #73.

[MoOx]

Released in 0.2.4