Add features for handling values

[mjackson]

It is very useful when writing a function that returns a promise to be able to return a value instead, in case you already have the value available. This way the caller can always treat the return value as a promise. So I've added a few "static methods" to the RSVP object that facilitate easier handling of already-resolved values.

RSVP.when

This function is the static equivalent of Promise.prototype.then. You can pass it a promise or a value, along with the usual done and fail callbacks.

RSVP.when(promise, done, fail);
RSVP.when(value, done); // Always resolves.

RSVP.resolve/reject

These are convenience methods for generating an already resolved/rejected promise for a given value.

RSVP.resolve(value).then(done);
RSVP.reject(value).then(null, fail);

RSVP.all

I also made a slight change to RSVP.all which goes along with these changes. It now knows how to resolve values as well as promises.

RSVP.all([ value, promise ]).then(done, fail);

[mjackson]

Oops. Just noticed that this pull request partially duplicates some work that @domenic did in #54. The only duplication is the part about RSVP.all being able to handle values as well as promises.

I think it just goes to show how useful this functionality can be. :)

[mjackson]

Closing this because the most important piece was already merged, albeit not from my work, and on second thought I don't think the other methods fit very nicely into the minimalistic approach of rsvp.js.

[domenic]

I think these are important. RSVP needs to fix its confusion between resolve and fulfill, but once it does, RSVP.resolve will be really nice. I've opened #60 for that. RSVP.reject on top of that would be helpful.

I'm not as sure about RSVP.when, since RSVP.when(x, f, r) can just be replaced with RSVP.resolve(x).then(f, r).

[mjackson]

My version of RSVP.resolve was actually more of a RSVP.fulfill. Your version of RSVP.resolve is just like my RSVP.when was. I was just using RSVP's existing terminology instead of using "resolve" and "fulfill" as they are defined in Promises A+. So I think we're in agreement.