#104 Have added the ability to select a specific TransferContentType currently only xml is implemented
using  class, but there is also a , which will be implemented in the next
commit
Added missing test class
Added a json serializer for sending json payloads
Move TransferUsing method to method param of WithPayload overload, defaulting to Xml
Add method allowing explicit setting of accept header used to request from the api, defaulting to xml
Removed Accept header specification as will do as a separate commit
Changed name to payloadSerializer to show that this is only for serializing Request DTOs
Removed empty ctor that reliued on the EssentialDependancyCheck. This kind of thing should really be handled by a consuming app either via newing up, or via an IoC framework.
Want to leave this as just basic initial configuration settings, i.e. consumer credentials and api endpoint for now, as there is an issue to remove the EssentialDependencyCheck method of wiring up for good
Changed method to expose PayloadFormat enum instead of IPayloadSerializer
Added Handles method to Serializer class so correct class is selected, and pushed a list of Serializers up to a field initialiser. Should be trivial to add this to a ctor at some point
Merge pull request #155 from gregsochanik/basic-json-support

Basic json support for payloads only