Basic calculate functionality

Addresses https://bugs.launchpad.net/oq-platform/+bug/1234765.

Hazard calculations work really well, and risk calculations require pre-computed hazard results to be available in the oq-engine(-server) DB. This is not an ideal long term solution, but at least it works.

Needs https://github.com/gem/oq-platform/pull/98, https://github.com/gem/oq-engine-server/pull/12, and https://github.com/gem/oq-engine-server/pull/11.