Explain plugging in to other Mocha instances.

See https://github.com/domenic/mocha-as-promised/issues/2#issuecomment-16109987.
Update/pin dev dependencies.
Add Mocha as a peer dependency.

Because of #6, require at least version 1.8.
Use shorter form for `"bugs"` field.
Make link to d98f2d9 more usable.
Add Promises/A+ logo.
Add keywords!
Update copyright year.
`"use strict"` everywhere!!
Version 1.3.0