Fixes for the SQL session.

I have corrected a few small spelling and grammar errors in the content of the SQL session. While preparing answers for some of the challenges I noticed that they are impossible to complete based solely on the information given in the notes so have added what's needed.

In our most recent Software Carpentry workshop one attendee had Python 3 installed through the Mac OS installer and the update to Yosemite had broken their Homebrew install. They asked whether the SQL session could be completed using Python 3 so I've updated the SQLite magic iPython extension to work with Python 3 while retaining backwards compatibility with Python 2.6 and up.