minor bug fix, remove moment dependency, remove callback

There was a minor bug where if you didn't use the expireTime in the params.

The moment library it a bit bloated and one of the nice things about this library is that it is nice and simple, so I've removed the dependence on moment.

finally, I've removed the need to get the signed url from a callback (in getSignedUrl), since the file read is sync there is really no need and it makes the lib easier to use the call, especially for signing lots of urls at once.