Added some convenience methods (which Q uses)

Hello there,

I wanted to replace q.js with a more lightweight alternative in a project I'm working on, so I picked ayapromise.

Unfortunately I needed some functionalities of q that ayepromise did not provide, namely checking the state of a promise. So I added three convenience methods used by q: isFulfilled, isRejected and isPending.

I also turned the ayepromise object into a function that wraps a value in a promise (or returns a promise), and added a method to check whether something is a promise or not: isPromise.

I added unit tests for all of the above.

Feel free to ignore the pull request. As I liked the project and needed those extra features I forked, but I understand if my changes are not in-line with the project philosophy and you prefer not to merge.

Anyway, thanks for the library.
Bye :-)