Add ability to configure services.
Throw exception for unknown configuration property.

An exception is now thrown if an unknown configuration property is
passed to the config method of a service.
Merge branch 'config' into develop
Introduce base class for test service.

TestService now derives from TestBaseService. This better reflects how
client code will actually work.
Merge branch 'base-service' into develop
Add HttpClientInterface and HttpClient.

Add interface and class for sending POST requets to eBay.
Merge branch 'http-client' into develop
Being implementing the sending of requests to the API.
Merge branch 'call-operation' into develop
Add production and sandbox urls for services.
Merge branch 'sandbox' into develop
Add building of ebay and http headers when sending a request.
Merge branch 'http-headers' into develop
Test that the correct XML is generated when sending a request.
Merge branch 'request-body' into develop
Create a TestHttpClient that can be used by other SDKs.
Merge branch 'test-http-client' into develop
Add test namespace.

Moved test classes into a 'Mocks' directory and added them to a mocks
namespace.
Merge branch 'test-namespace' into develop
Remove un-needed test class properties.