Added new command dev:log:db

This is a command that has come in handy for me a lot.  I tend to need to flip on db query logging maybe once a day or so when troubleshooting a query and wanting to quickly validate that the underlying SQL is OK.  And it became a big time saver for me - who wants to open up a Varien library file and modify two or three lines every time they want to see some SQL.

I know that there aren't really other commands in magerun that function by modifying source files, but I hope you might accept this pull request :)