Status and diff badges for builds and tests

This addresses the backend part of LbNightlyTools#20

Examples are here:

• http://rmatev04.cern.ch:8000/badges/lhcb-gaudi-head.2489/build.svg
• http://rmatev04.cern.ch:8000/badges/lhcb-gaudi-head.2489/build.svg?platform=.*gcc.*
• http://rmatev04.cern.ch:8000/badges/lhcb-gaudi-head.2489/tests.svg
• http://rmatev04.cern.ch:8000/badges/lhcb-gaudi-head.2489/build-diff.svg?ref=lhcb-gaudi-head.2488
• http://rmatev04.cern.ch:8000/badges/lhcb-gaudi-head.2489/tests-diff.svg?ref=lhcb-gaudi-head.2488
• http://rmatev04.cern.ch:8000/badges/notfound.12345/tests-diff.svg?ref=lhcb-gaudi-head.2488

Caching based on the CouchDB document ETag is implemented, so hopefully this should be light enough on the DB server (and on the frontend)

example embeded images