Added engine module. Moved Job.launch() to the engine.
Added engine module. Moved Job.launch() to the engine.

Refactored tests accordingly


Former-commit-id: f2a319080bb42487bec5b464ba1d21dd01e2bca1
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine