Add Cache-Control headers

Explicit no-cache for most routes.
1-hour cache for query route.
Add ETag for query route
Add method restriction to read/_status route

Removed HEAD from write/_status route, flask accepts HEAD on GET routes.
Merge branch 'master' into cache-control
Add etags conditional get

Also add feature tests for cache control.

@maxfliri
@robyoung
Add newline
Merge pull request #63 from alphagov/cache-control

Cache control