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

Added tests. Fixed few minor issues found while writing tests.
Fixes #5 
I'm not sure we should test it, since it's based on `_ok` which is private for HealthCheck class - not sure, but it sounds like `_ok` is something which shouldn't be private since it's used for custom checks.  The same for details.

not related to this pull, but I would make it more clearer - didn't expose private attrs for modification, but instead:
 - added internal boolean attr, like `_check_was_run` (or any better name)
 - made custom setter for `is_ok` and `details` to additionally set `_check_was_run = True`
 - made custom property decorator for both `is_ok` and `details` to check `_check_was_run`

Thus clients won't know anything about `_ok` and `_details`