Add initial cmake build support

Tested on cmake 2.8, on both mac os 10.9 & ubuntu 14.04.

Current shortcomings:
* No cmake 3 fancyness - but it works
* No "make install" support

Also note, I renamed setup-autotools.sh to setup-gtest.sh, since that's really all it does.
Should be `kj-async` (i.e. hyphenated), right?