Nick/week2 sql dataframe actions

[nickhepp]

• Merging the SQL and DataFrame API weeks into week 2
• Adding a stretch challenge with unit tests -- Question for everyone: do we want unit tests to be focused on for stretch goals for the students that are looking to learn more?

Note: I havent implemented the solutions yet -- I wanted to get feedback on what I've done before I've taken the time to write up the solutions.