Add x-app-usage response headers to result

[lendrom]

Add x-app-usage response headers to GraphAPI request result
Facebook API returns x-app-usage header which contains json with percentage usage of the api for various metrics. It would be handy to have access to that data when using this facebook-sdk.
My simple pr just checks for the header in fb api response and adds it to the result returned by GraphAPI instance if available.

FB docs:
https://developers.facebook.com/docs/graph-api/advanced/rate-limiting
It was also reported here:
#314

[martey]

Thanks for the pull request. Would you be able to update the changelog and add a simple automated test?

[martey]

I've also made changes to the CircleCI configuration so that the automated tests won't automatically fail.

[lendrom]

I can add some tests, but I can see the tests are running on the actual FB API. Wouldn't it be better to run them on a mocked version of it or use something like pyvcr? Now I can reach my app's limits just by running the tests.

[martey]

Would it make more sense to just return all headers? This would negate the need to add more code if Facebook adds more useful headers (e.g. X-Page-Usage, etc.).