minor typo fixing in the readme
Added test-suite
Added a test for erronous strategie output.

The output of the play function is always evaluated as a boolean:

- "", None or nothing will always evaluate as False, hence defect.
- a number or a string will always evaluate as True, hence cooperate.

Note that if the play function is not defined, the play function in the generic
player class returns nothing.

closes #2 (wontfix)