Mod0

Changes made:

- In the earlier version, I had a many-to-many relationship between categories and videos. Changed to 1:M
- written tests for video attribute validations (title and description) using shoulda-matchers
- written tests for video/category association - 1:M - using shoulda-matchers
- removed "render :show" from the videos#show based on feedback from Albert (alagram)
- added search_by_title method to Video using TDD.