Additional RSVP imports

[Turbo87]

Right now it is only possible to

import RSVP from 'rsvp';

but to get to Promise or resolve() it is not possible to

import { Promise, resolve } from 'rsvp';

IMHO we should change that

[locks]

Not a bad idea, indeed.

[rwjblue]

Seems like a dupe of ember-cli/ember-cli-shims#67 which @stefanpenner opposed 😕

[cibernox]

FWIW, I think that it makes sense to expose Promise directly, without destructuring RSVP. However, since Promise.resolve is part of the spec, I don't see the case for RSVP.resolve, unless there is some subtle difference I'm not aware of.

[rwjblue]

Some of this is done, but as I mentioned in #18 we should likely bring in a few more named exports here.