Run calculations via REST API (initial sketch)

This is for https://bugs.launchpad.net/oq-engine/+bug/1206913

This branch is basically a first sketch of the functionality. The hazard calculation part works more or less, but the risk part is not implemented yet. I just wanted to submit what I have so far for review.

https://github.com/gem/oq-engine-server/pull/5 needs to merge first for a clean diff.

Needs https://github.com/gem/oq-engine/pull/1240 to have the proper oq-engine API functionality available.