Use the correct constant for OsType.DESCRIPTOR (ie. the constant used in psdparse: http://telegraphics.com.au/svn/psdparse/trunk/descriptor.c)

This requires the signature of decode_descriptor to match the other decode methods.
The unit key for unit floats is 4 characters, not an integer
Merge pull request #7 from makielab/fix_descriptors

Fix descriptors