Add a query for checking replication slave status via Slave_running variable

I am currently using this to graph and alert on slave status. This variable should be reliable enough on more recent versions of mysql to ascertain slave status. 

I looked into grabbing Slave_IO_Running and Slave_SQL_Running, but the show slave status command does not allow isolation of those variables so we would have to manipulate the text post query which would have been a deeper change.

This also might help address issue #425