Nutter

Enhancements/Fixes:

- Parallel execution of test fixtures.
- Ability to pass custom parameters from the CLI tool to notebooks

Closes:
66, 61, 72 , 58 , 46

Enhancements/Fixes:
- Polling interval for notebook execution status check has increased from 1s to 5s by default
- New flag controls the polling interval: 'poll_wait_time'.
- INVALID_STATE is retried when returned by the API.
- Test cases can be written as *****_test.py, thanks to quanuw

Related issues/items:
48 , 50 , 16 , 41 , 49

- Support multiple assertions for a test.
- The run_ is no longer required for test cases.
- You can create multiple assertion_ methods and they will be executed after the before_all method, where you can run your notebook under test.
- Documentation updates.

Closes 40 39 35