Add Point and Millimeter types to UnitFloatType

See http://forums.adobe.com/message/5399877#5399877
#Pnt and #Mlm can occur as Unit Float types.
Unfortunately I'm unable to supply an example PSD b/c I don't have Photoshop and can't distribute the PSD I encountered #Pnt in, but this makes the library work for me. I threw in #Mlm because why not :)