Fixed the granulepos calculations.
Fixed the granulepos calculations.

The granulepos should use a fixed sampling frequency of 48 kHz and not
the current Encoder sampling frequency.

Also removed the granule information on the header fields. As the
granulepos tracks the audio PCM samples, there's no use of it on the
Opus Headers.

Fixes #40

Rantanen/node-opus,Rantanen/node-opus,Rantanen/node-opus,Rantanen/node-opus,Rantanen/node-opus