Change 'w' to WRITE in error messages
Move constants for soundfile string out of SoundFile class

All constants are now in one place

In addition, _getAttributeNames() was added which helps auto-completion in
IDEs.
Fix __setattr__()

In new-style classes the base class __setattr__() has to be called
instead of inserting the value into self.__dict__.

See http://docs.python.org/2/reference/datamodel.html#customizing-attribute-access
Change public attributes to properties
Fix a seek-related bug