Add SimpleCov

Add SimpleCov and configure it so that it doesn't reports coverage when
`rake` completes and acts only as a 'red light' if coverage drops
significantly (by 20%) or drops below a percentage (80%).

Note that to silence the 'Coverage report generated for' message that
reports the coverage percentage when SimpleCov completes, we
monkey-patch the SimpleCov::Formatter::HTMLFormatter class.

The SimpleCov::Formatter::HTMLFormatter#output_message _only_ prints the
'Coverage report generated for' message, so by monkey-patching it we
don't lose the other warnings that we want to keep (mentioned above).