Add a server for histogram bucket ranges.
Better error handling
Cleanup
Merge branch 'master' into histogram_server
Serve either raw Histograms.json or bucket ranges.

Raw Histograms.json:      /histograms?revision=url
Histogram bucket ranges:  /histogram_buckets?revision=url
Merge pull request #7 from mreid-moz/histogram_server

Add a web server for Histograms.json and Bucket Ranges