Make response headers available to caller

[charlieok]

This is especially useful for the rate limit headers returned by fitbit's api
https://wiki.fitbit.com/display/API/Rate+Limit

Getting Fitbit's rate limit information to the client application allows the application to better schedule calls to Fitbit's api.

[whazzmaster]

I like it; I'm going to play with this patch locally for a bit before merging as I want to think about how the http_headers key addition will work out in practice. My initial reaction is that it's a good backwards-compatible step, but I want to be sure.

Thanks for the PR!

[charlieok]

Great, thanks for taking a look! Looking forward to the day I can delete our local fork 😄

[jamescook]

👍