Rewrite the introduction. Closes #106, closes #95.
Update differences document for recent changes and clarity.

- Move `promise1 !== promise2` non-requirement to "Omissions" from "Additions".
- Add `onFulfilled` and `onRejected` called as functions requirement (new in 1.1) to "Additions".
- Change "returns a promise" to "returns a thenable", and expand upon what has been added (i.e. the resolution procedure).
- Use "reason" instead of "rejection reason".
- Rewrite the "Clarifications" section for better structure and clarity.
- Add "thenable" to the list of things we clarify.
Changelog should reference newer [[Resolve]].
Add @ForbesLindesay and @erights as contributors.