Update search spec

This adds:

- Semesters to Course datums, since Courses taught in different
  semesters would be impossible to distinguish otherwise
- Aliases to Course datums, since a result may match a query on an alias
  other than its primary alias

This removes:

- Departments from Instructor datums, since having them would be
  computationally expensive (several joins) and the value is not obvious
Merge pull request #1 from pennappslabs/search

Update search spec