Added the AWS signature test suite, which is presently failing

What started as me trying to use this library to do some S3 benchmarking turned into an epic debugging session. I'm getting incorrect signatures generated for S3 and the other services I tested. 

I decided to add the signature test suite from AWS in to see if they matched up, they do not. The first issue that jumps out is the incorrectly formatted Date header that gets added. The signature spec is confusing on the formatting of that header, but the http spec isn't. It should be a `http.TimeFormat` not a `iSO8601BasicFormat` I believe.

I'd love to know how this worked in the first place? @bmizerany I presume you've been using this without issue?

I'm running `go version go1.3 darwin/amd64`.