Add topic_id column to resources, pass test 1
added name column to topics, validated name in topics
Add opinion column to topics, validates presence of opinion
added length validation for opinion in topics
Add url column to resources
added validation of url in resources
Add validation of presence of url to resources.
Add validation of numericality of topic_id in resources.
added validation for presence of topic_id in resources
added difficulty column, validated difficulty values in resources
Implement add_resource! in topics
Merge pull request #1 from anniesing/level-1-tests

Level 1 tests