Use consistent logging and document how to use it

[sevdog]

Currently inside this package there are few methods which has a logger enabled. Some are configured as getLogger("health-check") while other as getLogger(__name__). This may cause confusion when someone has to configure logging since he will expects a package to log under a single main logger while current configuration leads to two loggers: "health-check" and "health_check", one with the dash and the other with the underscore.

It would be better to have a single logger, to ease configuration for other projects which use this as a dependency, and only where it is used (since some files declares a logger which is never used afterwards).

Also currently there is no documentation regarding how to use the "health-check" logger, which would be very useful to know.