Allow ES6-style Promise construction

[andrewimm]

Allows Parse.Promises to be constructed in the ES6 "executor" style, without intervening with old construction.

// Valid:
return new Parse.Promise((resolve, reject) => {
  myAsyncFunction((result) => {
    resolve(result);
  });
});

// Also still valid:
let p = new Parse.Promise();
myAsyncFunction((result) => {
  p.resolve(result);
});
return p;

cc @grantland

[andrewimm]

Part of the launch checklist for #136

[andrewimm]

Also added the following static constructors from the ES6 spec:

Parse.Promise.resolve(value);
Parse.Promise.resolve(thenable); // Chains the new promise to the end of the old one, essentially a noop
Parse.Promise.reject(error);

[peterdotjs]

can we use executor as param name for consistency?

[andrewimm]

👍 absolutely. I was calling it "closure" before I read the spec and determined it was deemed an "executor"

[peterdotjs]

LGTM.