during & doDuring with async test

[uzyn]

It's basically whilst and doWhilst with async truth test.

Addresses #156 #602

Will add documentation if maintainer is okay with this PR.

[aearly]

👍, since several people have asked for it.

Also -- I wonder if the test should be passed a callback of the form function (err, truth) {...} rather than just function (truth) {...}. I realize that most of asyncs predicate-accepting functions expect a callback with just atruth` parameter, but it's something we're going to change in async 2.0.

[uzyn]

Awesome. Let me update that and the documentation as well.

[uzyn]

Alright. Updated as suggested.

Thanks @aearly