Use must_be_instance_of instead of must_be_kind_of
Make AnalysersRunner return smells instead of smell_adapters

Since AnalysersRunner was coupled to SmellsAggregator, it made sense to
move SmellsAggregator inside AnalysersRunner.
Rename variables
Rename test folder and files
Add Churn class

It's so simple to calculate churn, there's really no need to depend on
the churn gem.
(For now. If there ever is a need to calculate churn in other source
control systems, maybe it would be better to use that gem.)