Milliseconds in timestamps

The Oauth1HttpMessagehandlerBase makes timestamps in seconds since 1st jan 19770. I have come accross one Serviceprovder that requires these timestamps to be given in milliseconds. Since the MessagehandlerBase does not allow for easily extension in regards to making timestamps beacuse the method is private and cannot be overriden in derived classes, I have made a small change to allow for any timestamp encoder. This fixes issue #322