A simplified co implementation only working with thunk

usage:

var co = require('./co');

function readFile(filename) {
	return function(callback) {
		require('fs').readFile(filename, 'utf8', callback);
	};
}

co(function * () {
	var file1 = yield readFile('./file/a.txt');
	var file2 = yield readFile('./file/b.txt');

	console.log(file1);
	console.log(file2);
	return 'done';
})(function(err, result) {
	console.log(result)
});