\Fitbit\Exception was unused. Remove modified params, message, and error code handling.
Separate Fitbit service creation and API calls.

* All user-related API calls go in `UserGateway`, all Food-related API calls go in `FoodGateway`, etc.
* `ApiGatewayFactory` handles setup for the Fitbit service, its dependencies, and provides access to the different gateways.
* Removed reliance on PHP's `$_SESSION` variable in the library itself. Can still use the Session OAuth storage provider, but the library no longer implicitly assumes that it's the one that will be used.
Delete `\Fitbit\Response` as it's never used.
Update composer.json.

* Add authors
* Change PHP version requirement.
* Point to my (popthestack's) repo.
* Point lusitanian/oauth to latest library version.
Fix GET param handling in `EndpointGateway::makeApiRequest`.

Fix GET request with query string in `FoodGateway::searchFoods`.
Update README descriptions and examples.