Added tests. Fixed few minor issues found while writing tests.

Added tests. Fixed few minor issues found while writing tests.
Fixes #5 
```
...I would make it more clearer - didn't expose private attrs for modification...
```
They're not exposed. The HealthCheck subclass has to set protected attributes of a check. And these attributes are not going to be accessed from outside - we have `is_ok` and `details` for this.