Added example_read_mac to Rhino3dmIo

In addition, modified rhcommon_opennurbs.xcodeproj to target two
different platforms (iOS and Mac OS X) to build the native libraries
for iOS and OS X respectively.  Modified build_mobile.py to the more
general build_native.py.  Modified on_tree.cpp in keeping with internal
SVN revision 101847 (in 6/trunk/).