Signed query

Pass the signature as a [query parameter](http://s3.amazonaws.com/doc/s3-developer-guide/RESTAuthentication.html). Takes a `time.Time` parameter. Don't love the SignURL name - SignQuery?

Duplicating writeSigData with writeSigURLData - could fold these in to the same routine, and check for whether a time value got passed in and the either do `data' or `expires`.

Not sure if this works. Also needs tests.