Status endpoint to check data freshness: /_status/

Currently checks:

- Tide predictions extend longer than 30 days into future
- Observations are no older than 60 minutes

We are using an HTTP 500 "Internal Server Error" status code to signify
when the API status is "bad". See discussion here:

https://stackoverflow.com/questions/25389261/which-http-status-code-should-i-use-for-a-health-check-failure

https://www.pivotaltracker.com/story/show/76363838 [Delivers #76363838]
Merge pull request #72 from sealevelresearch/add-status-endpoint

Status endpoint to check data freshness: /_status/