OSX: Upgrade to OSX 10.10 Yosemite SDK / Xcode 6.1.

Use the 10.10 SDK now, building for 10.8 - 10.10.
Use new Xcode 6.1 compiler toolchain.
Test rebuild of all mex files shows it working.
Bump version of officially supported OS X to 10.10.

-> Rebuilt Screen to display new support status.
-> From now on, only OS X 10.10 Yosemite will receive testing or bug
fixes or any other sympathy.