Fix type in doctest from README

Doctest interpreter output incorrectly defined for use of multiple
decorators.
Add missing unittest for long conversion

Automatic conversion for long objects in Python2 was not being tested.
Adds a unit test for those versions of Python where long is supported.
Automatic type conversion with begin.start

Enable begin.start to wrap the decorated function with begin.convert to
automatically convert basic types. Closes #12.