Use global Promise

[statianzo]

Expect a globally available Promise either natively available or
provided through a polyfill.

.always is removed from the promise api as it's non-standard. Related
tests are removed.

Given that resolve and reject only accept one argument, responses
are passed through as {data: parsedResponse, xhr: xhr}. Errors
provided to reject have the same interface.

Added es6-promise as a devDependency because PhantomJS doesn't have
native promises yet.

Opening this as a discussion point for #20.

[coveralls]

Coverage remained the same at 100.0% when pulling 2292b91 on statianzo:promises into f5c6d94 on fdaciuk:dev.

[fdaciuk]

Nice work @statianzo! Thanks ✨
I'll try it today, before merge.

Thanks again =)

[statianzo]

I'm still not certain on the shape of the response object. Looked at the visionmedia/superagent Response object and they have a few convenience things.

{
  xhr: xhr,
  req: req,
  text: xhr.responseText,
  body: parsedBody,
  status: 200,
  statusText: 'OK',
  //...
}

Also, if you plan on merging, I'd be fine with updating the docs first.

[fdaciuk]

Yes, we need to think about the returned object.
Sorry, but I'm really busy this week. I'll try to review this PR in the next week, ok?
Thanks for all job, for now =)

[fdaciuk]

Hey @statianzo! Sorry for a so long delay =)
I'm working on a ES6 version for this module, and I'll use Promise on that.. This way we'll can use both: generated ES5 version for simple projects, and ES6 version when we use webpack or something like that to compile =)

Thanks for suggestion and all work on this! ❤️