Code reorganization

[ryanmark]

Made a bunch of changes to try to simplify stuff.

• The KintoObject does much more now, KintoServer, KintoCollection, KintoBucket and KintoRecord all inherit from KintoObject. This eliminated some duplication from these classes and from the base KintoClient class which was doing what KintoServer does now.
• Almost all requests now use the KintoRequest object. While this makes the single requests more complex, it unites the code paths between batch requests and single requests.
• Renamed the send method in KintoRequest to execute. send is a core ruby api method.
• KintoBatchRequest now inherits from KintoRequest. All request objects have an execute method.
• KintoObject info responses are now cached. This will probably cause some trouble checking if something changed the data on the server outside of the object instance. Added a reload method to KintoObject to bust this cache.
• Added a rubocop config and fixed most rubocop warnings and errors.