V0.9.x - Allow multiple forks to run in parallel and provide a counter to each fork

Address #164

- Do not abort a run if another run is wanted.
- Do kill all forks if spork is terminating
- Provide Spork.run_count so each process know some thing about it. This allow the rotation of database connections for running test in parallel (I'm working on this: https://github.com/phuongnd08/multi_spork)