support earlier OS X versions in wheels

Wheels built for 10.6 (e.g. Python.org) work on later OS X (e.g. System Python).
The reverse is not assumed.
test OS X arch support
support intel and universal multi-arch tags on OS X

- intel == x86_64, i386
- universal == x86_64, i386, ppc, ppc64
test multi-arch wheels on darwin