Expire caches in merge API

Merge API did not expire caches causing changes not to be shown.
Return JSON payload also from merge API

Both import and update API endpoints return {'ok': 1} if all went well.
For consistency return such message also from the merge endpoint.
Catch timeout errors from external services

Network requests to external services (Bugzilla) may time out. Catch
those and handle as not found instead of rendering an error page.
Run external service calls in parallel

When requesting data from multiple external services each http call
blocked the execution and thus caused the response be slowish. Now the
external service handlers are classes including Celluloid, and the
controller collects a list of futures. This way all the handlers are
launched in parallel which will speed up the response.
Merge branch 'dev'