Make assessment.assessment_category a required column

The addition of the "Other" assessment(via a data migration in this change set) will account for any existing assessments that have a null assessment_category field. This change will allow the UI to make category a selectable field for all pathogenicities instead of the limited subset as it is now.
Fix tests to account for required assessment category
Add importlib as dependency in python 2.6 environments
Update UI to include "other" category & always require category
Merge pull request #79 from cbmi/issue-76

Make assessment.assessment_category a required column