Modifications to Q.async

[sjmeverett]

This pull request supports the following use cases:

• the ability to call Q.async on any function, without caring if it is a generator function or not: if it is a regular function you get a promise for the return value, if it returns a promise you get that promise, and if it is a generator function it behaves like the original Q.async
• the ability to just type yield anotherGeneratorFunction() instead of yield* anotherGeneratorFunction() or yield Q.async(anotherGeneratorFunction())

[kriskowal]

I try to keep concerns separate. I’m sympathetic to the case for convenience, but this comes at a high cost in clarity. There is Q.promised(f) (among other things) for the one case, and Q.async(g) for the other.