Added get_rate() and get_channels() to the PCM object.
Changed the naming of the PCM sample format identifiers to not have the
prefix PCM_FORMAT.  This could likely be moved to a module level
dictionary that holds all these values.  In addition added support for
the IEC958_SUBFRAME_{L,B}E, SPECIAL, {S,U}24_3{L,B}E, and
{S,U}20_3{L,B} sample types.