Add a startup time check that all MySQL tables are set to UTF-8 encoding.

If any non-UTF-8 tables are found, log a warning message (showing the affected
tables) and refuse to start the backend.

Added a new configuration option to allow sufficiently foolhardy people to
override this check and start up anyway.
Merge pull request #29 from hudmol/check-db-utf8

Add a startup time check that all MySQL tables are set to UTF-8 encoding