Mocha module detection

Support for loading mocha-as-promised when running outside of the mocha command line. Search modules starting at require.main to find the first module ending in mocha/lib/mocha.js, and assume that it is the running mocha module. Tested this technique on windows and linux, under grunt-mocha-test, grunt-mocha-hack and grunt-mocha-spawn. I'll keep trying to poke holes in the idea, but I think it should work. 

PS thanks for mocha-as-promised, it cut lots of code out of our promise-based tests. Hopefully it makes its way into mocha someday. Have a good time at JSConf.