Changing _compose() to support promises.

Compose would now handle promises returned by any of the composed functions. The resulting function
will evaluate to a promise if any one of the composed functions returns a promise. If all of the composed functions return something other than a promise, then the behaviour of compose() is the same as it was before. In this sense the change is fully backwards compatible.

I added one new test and modified two tests to test promises. I added Q to dev dependencies to test promise support. However, Q is not necessary at run time, and this implementation should work with any promise implementation.

See https://github.com/ramda/ramda/issues/512 for discussion.
then should at least be a function