Deprecate github.com/pkg/errors

• Remove the usage of github.com/pkg/errors, in favor of std library errors and fmt packages
• Added ErrNotRunYet sentinel error

Improved error handling for check registration

Merge pull request #51 from dmarkhas/sensible_defaults

set default execution period to avoid panic

Major API improvements

• Added proper context support
• Added functional options API
• Refactored packages for better structure and removal of cyclic deps
• Maybe more (I'm not organized enough :P...)

Extract metric to own module and introduced health and check listeners

Metrics are now reported via listeners to avoid forcing a dependency on the users, and to better support other metrics libraries