Add Overall Uptime across all hosts to API#670

Hello,
Please considering adding the overall uptime across all hosts to the API, this is available on the status pages and would be great to grab as a ‘single value’ from the API to show our overall uptime across the whole monitored platform.

having the 24 hour, 7 day, 30 day and 90 day values would be awesome.

Thank you for considering.