Bug 865553 - Handle MPEG version 2 packet lengths. r=padenot, a=akeybl,webaudio

Previous code didn't adjust bitrate and scale factors correctly
for computing frame sizes on mpeg 2 and 2.5 files. See
http://blog.coryhill.net/2009/06/calculating-mp3-frame-length.html
for the extra factor of two.

Removes a case for handling length calculation for layer-1 files,
which we don't support.

Adds a tagless mpeg-2 test case for the new sniffer code.