OTHER: Cleanup

Re-enabled nellymoser (ffmpeg appears to be okay with it now)

Fixed possible infinite loop in the code that handles the data (if any)
between the header and tag data.
OTHER: Rename constants to something more readable
BUG(2276): Set duration proper

This adds an AMF reader which allows us access to
onMetaData's duration object and lets people see some interesting info
in the log if they ran xmms2d -v.

Also cleaned up byte swapping functions.
BUG(2276): Check if duration is already set
OTHER: AAC support

flv now supports AAC (thanks to juhovh for reminding me
that I changed some _peek calls to _read)

Split header reading code into another function. A nice side effect
of the AAC support.