Closes #1.

• This change is most easily reviewed using Git's -b option to ignore whitespace.
• Our code styles differ but I tried to mimic yours as much as possible.
• As discussed in Slack, result headers are omitted from the Promise API. We could resolve the promise to an object containing both results and headers, but this would differ from the current callback usage and normalizing the two would mean a breaking change to callbacks.
• I didn't want to clutter up the README too much but still felt this needed documenting, so I added one simple example. Let me know if you prefer something else.