Add status endpoint

@jh-bate @kentquirk 

Needed a status endpoint on tide-whisperer.  This is a quick pass at it.  It's working locally, pls review and merge.
I know it's boring but I would like to standardize the responses we use for status checks. Although it's not what I implemented in other systems, I like what you've done here for the error code (return 500 and info). I'd like to say that a 200 response also includes something standard. Perhaps just "OK\n"? (Is there any use case for including additional information, like a link to tidepool.org or version information?)