minor refactor of iao.owl

[Alan Ruttenberg]

I have made the top level iao have just metadata and imports and moved
the class and property definitions to iao-main.owl
This based on a request that there be a way to import iao without
pulling in the obsolete classes. To do so one can import
http://purl.obolibrary.org/obo/iao/dev/iao-main.owl

OBI users may want to svn update.
-Alan