More tostring abstraction

added on() and off() functions - quick functions to turn on/off SQL
execution.

tostring() now optionally takes a callback function, if so, it'll
execute it, and return the stored query. example:

$x = $db->tostring(function()use($db){
$db->rows('pudl_user');
});

string(26) "SELECT * FROM `dbtbl_user`"