Made 'season' optional for stats calls

API documentation states that if 'season' is not specified, stats for
the current season will be returned. Added methods which do not require
a 'season' parameter for stats calls to allow this.

Modified existing unit tests to give coverage to calls where a season is
not specified.
Added tests for stats calls without 'season'.
Merge pull request #17 from t-davies/master

Made 'season' optional for stats calls