Initial atlas datasource

Code copied from https://github.com/grafana/grafana/pull/1265
Minimum interval enforcement
A stub to support eventual PNGs.
Move atlas data source, add some tests around png url generation
Add a super-basic editor component
Remove stray .only
Feed error inspector when we have a query failure.
Merge pull request #3 from matschaffer/atlas-datasource

Initial atlas datasource