Enable use on Mac OS X 10.8.x with new method signature & dummy implementations.

Without this patch 10.8.2 does not work--contrary to what our README for libpebble suggests.

I'm proposing pulling this into master on the assumption that 10.8.2 & 10.8.3 are the most common 10.8 versions in the wild so we should support them out of the box.

For more details see the commit message.