Add method_or_response= as optional keyword parameter for add() method.

[paulsuh]

Fixes issue #734

Adds a keyword parameter method_or_response to the add(...) method. The new keyword parameter is added as the last keyword parameter so that existing code that treats the first and second positional arguments as being assigned to the parameters method and url respectively continues to work without modification.

Unit test of the new functionality (all tests passing) and documentation in the README.rst have been added.

[beliaev-maksim]

please also add a change explanation in the changelog

[paulsuh]

@beliaev-maksim Change log updated