Will/ai grading xml rules

[TIM-589](https://edx-wiki.atlassian.net/browse/TIM-589)

* Provide a default value for the algorithm ID
* Validate that the algorithm ID is set to "ease"
* Validate that example-based assessment does *not* provide start/due dates
* Support just AI configuration (no self- or peer-assessment)
* Validate example-based assessment criterion options have distinct point values.
* Validate that example-based assessment provides at least 1 training example.

I also refactored the tests a bit and added some tests for example-based assessment deserialization.

@stephensanchez @gradyward