Signed urls

Methods:
  * `buildUrl(method, arg1, arg2, ...)`, and
  * `buildSignedUrl(method, arg1, arg2, ..., {expiration: 60 * 5})`

The method `buildSignedUrl` only works with `GET` requests... but all in all this pretty nice...
Give it a quick review and let me know if you have concerns.

To support signed URLs we need to upgrade to newest version of `taskcluster-base`, I've already started this for `taskcluster-queue`, basically I want signed urls for getting artifacts :)
They work like a charm for this as they redirect with another signaure (S3 or SAS) to the actual artifact... It's beautiful :)