initial yield object support
make co.join() private + refactoring

renamed to `arrayToThunk`. object version is `objectToThunk`.
cleanup tests.
npm test
benchmark: add `object`
test: add a recursion test

we may probably want to add more later
fix copy

need to update README still
Revert "you never use toString() anymore"

This reverts commit 2a94e822b0c66cfdbba21d521ea1f0f6602eb281.
Merge pull request #42 from jonathanong/obj

initial yield object support