Added slides for introduction to SQL.

Following Issue #559, I've created introduction slides for SQL. The presentation currently runs ~4min30sec, and can be split in half if it needs to be shorter. In comprises an overview of the topic, followed by a short motivational example.
I was just having trouble remembering where to advance the slides for each new bullet point -- the "[Click]" phrases are just presenter directions to advance the slide forward one "click" : )