PROTON-781: Created a utility module for the Ruby Engine APIs.
PROTON-781: Added the Error class to the Ruby bindings.
PROTON-781: Added a constants value mixin to the Ruby bindings.

The purpose of this mixin is to make it easier to create classes that
represent constant values from the Proton C code.
PROTON-781: Created the new engine directory for Ruby APIs.
PROTON-781: Added DispositionType to the Ruby bindings.

It encapsulates the statuses of received, accepted, released, rejected
and modified disposition types.
PROTON-781: Added the Disposition class for the Ruby bindings.
PROTON-781: Added the Condition class to the Ruby bindings.
PROTON-781: Added the Connection and ConnectionState classes to Ruby.
PROTON-781: Added the EventType class to the Ruby Engine APIs.
PROTON-781: Added the Event class to the Ruby engine APIs.
PROTON-781: Added the Collector class to the Ruby engine APIs.
PROTON-781: Created the Endpoint mixin for the Ruby Engine APIs.