Parse courses from formated text file

- Define rake task courses:parse
  - Parses file at /lib/assets/courses.txt
  - Creates courses in db
  - Outputs invalid courses to /lib/assets/invalid_courses.txt
- Call feeds:parse from db:seed
- Git ignores invalid course list
- Mention rake tasks in README
Add course validations
List courses at /courses
Add offset and limit to course index
Use faster json backend
List distinct departments at /departments