PROTON-781: Re-arranged the Ruby classes to separate Messenger.

Move the simple messenger related code into a directory named
"messenger". All the rest were moved to a directory named "core".
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 the Disposition and DispositionType classes to the Ruby engine APIs.
PROTON-781: Added the Condition class to the Ruby engine APIs.
PROTON-781: Added the Connection and ConnectionState classes to Ruby engine APIs.
PROTON-781: Added the Event and EventType classes 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.
PROTON-781: Added the Container class to the Ruby engine APIs.

Also includes supporting classes for a priority queue, events and
timers.
PROTON-781: Added Ruby examples for the Reactive APIs.