Moved Accept and added Content-Type header

Since I do not expect anything other than JSON is used I
have moved the accept and content type headers into a
generic location.
Add Event Entity and mapping

In this commit we add an Event entity that will be used to catch the
contents of event-related responses. Associated with it is a mapper
that can convert an associative array representing the Event entity
into an Event object.
Map responses to Entities

This commit introduces a special Response object that contains the
resource representation (which is an object if a mapper is defined
in the Description's model section) and the meta section.

With this mechanism it is trivial to add a mapper and have the
returned associative array transformed into a real entity.
Add event listing to the Event endpoint

In this commit we add the ability to retrieve a series
of events based on filter values.
Update example with the latest changes

The instantiation of the Webservice Client (as called by Guzzle) is
moved inside the Joindin Client to provide a simpler workflow; and
to be able to inject the event listener there that maps responses to
entities.

As such the example is updated to reflect the new situation.