Use when/lib/Promise directly to avoid double-browserify problem

This allows most.js to be browserified either standalone, or as a part of a larger application.  We can't yet implicitly depend on global `Promise` because the native impls are still slow and/or buggy (or nonexistent).  This keeps the same functionality as before, but just makes it possible to use browserify.

Also, removed the public `most.forEachUntil` and `most.observeUntil`.  Their implementations had been removed a while ago, but I forgot to remove the properties from the main module's exports.

Fix #42