B3 database tables check (mysql only)

This pull request carries an additional check which verifies that B3 database is non empty (mysql only, since SQLite already create tables when they are missing). Whenever B3 database is found empty, B3 stops and print a message in the log file which explains the problem and how to solve it.

This is mostly due to users who have difficulties in reading a log file (which already explains the problem but doesn't give a solution). As an example: http://forum.bigbrotherbot.net/cod6-(mw2)/starting-plugins-223/