Use MySQL in development

sqlite3's behavior differs enough from MySQL's that it's not a good idea
to use sqlite3 for backend development, since what works in your development
environment might then not work on the real servers using MySQL.

Update the documentation and sample settings file to use MySQL.
Mention that some OS distributions provide MySQL packages
Merge pull request #694 from dpoirier/bug-932950-migrate-from-scratch

Bug 932950 - Use MySQL in development