splitted Payment into a endpoint and model structure

[tubbynl]

red (non-compiling); example splitting the responsibilities of model and endpoint into own hierarchy’s,
this example code for issue #93

currently due to static methods the sdk/api is limited to single-context use and using it in tests is harder.

this example code is illustrates the following

• separating the model- and endpoint concerns into own hierarchy
• enabling non-static access on the endpoint methods

it is however breaking api, up to some extend it is possible to prevent this, but having model and endpoint concerns in the same package/class (for Payment at least) is not splittable without breaking compatibility