Implement RIFF file management

+ Add Crash.Audio.RIFF
+ Add Crash.Audio.RIFFData
+ Add Crash.Audio.RIFFItem
+ Use RIFF classes for WaveConv.ToWave
Use better checking code for argument validation

+ Simplify an exception message
Move ADPCM code into an ADPCM conversion class

+ Add Crash.Audio.ADPCMConv
+ Change Crash.Audio.SampleLine.ToPCM to use ADPCMConv
+ Update COPYING
Fix ArgumentNullException throws