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 didn't actually know about pandas.read_sql_query, but reading up on it now, I think that pandasql, like R's sqldf package, sets up an SQLite database automatically in the backend. So no actual database connection is required; you can query dataframes that aren't stored in a database anywhere. Is that the same functionality as read_sql_query, or does read_sql_query require an existing database connection?