Add convenience method to strip promise methods from new refs

[wescravens]

Currently new refs created with the push method cannot be resolved in promise chain. Since the return from .push is an object with the prototype of Fireproof and a .then method, returning it from a promise callback will result in it being treated as a promise thus resolving with undefined since .push doesn't have a resolve value.

Temporary work around:

Fireproof.prototype.getRef = function () {
    let ref = new Fireproof(this._ref);
    delete ref.then;
    return ref;
};