Allow to use native promises instead of RSVP

[mydea]

I initially tried to get it to work with using require('rsvp') when building with Embroider.
However, I couldn't get it to run. I tried adding rsvp as a dedicated dependency, as well as ember-auto-import, and importing RSVP in an initializer, but nothing made it available under require('rsvp') in my embroider app.

I ended up adding an option that allows to opt out of using the RSVP wrapper at all. I think this is an acceptable and backwards compatible opt-out option that should be OK for apps that target only browsers that do not target IE11 anymore.

Let me know if you think this works, or if a different approach would work better.

Closes #622

[rwjblue]

Seems good to me overall. I'd like to make a follow up after this lands to issue a deprecation when not using preferNative and nativePromise options.

[mydea]

I have removed the var Promise = originalGlobal.Promise part, and also added a NATIVE_PROMISE=true yarn test step to travis to make sure this is covered.