Fix rendering under iOS7 with 64-bit without breaking backwards compatibility.

Uses -drawWithRect:options:attributes:context: and -boundingRectWithSize:options:attributes:context: when they are available, falls back to the iOS6 methods if not.

Tested using the demo app on iPhone/iPad simulators under 6.0, 6.1, 7.0 and 7.1b5 and on device under 7.1b5.