Add a conformant implementations list.

Closes #59, closes #57.

In the 1.1 gh-pages update, we'll put a link to this in the footer.

People should feel free to update their descriptions; I mostly pulled them from GitHub and from the top line of the readmes.

Should we have some way of distinguishing the "popular" promise libraries? E.g. over 100 GitHub stars? I'm thinking of this from the perspective of someone reading a fun promises article, seeing a link to the Promises/A+ spec, clicking over to implementations, then being confronted with a baffling array of choices.