Split 08 SQL notebook into two sections

The current notebook combined adding and dropping tables and changing
the data in existing tables into one long section. These are logically
separate sets of commands and grouping them together created a bit of a
wall of text.
Add Update to SQL lesson on creating and modifying databases

When showing students how to add and remove records it will be natural for
them to ask how to modify an existing record. This demonstrates how to do
this using Update in a few lines.
Add updated markdown version of 08 SQL notebook
Merge pull request #721 from ethanwhite/sql-create

Add UPDATE to SQL lessons