Add backtrace information to database debug log

This will pass  the backtrace information to any attached logger when executing a database query in debug mode.

Currently only implemented for `Mysqli` and "`Pdo`" drivers.

An example implementation might render like this:

![issues list 2013-10-03 13-36-10](https://f.cloud.github.com/assets/33978/1264091/ca7cf430-2c5a-11e3-9e95-05bc29abedee.png)