Implemented logging of flushes + option to enable it in configuration

Hi Turpentine Team,

On several of my client's installations they asked me questions about why sometimes pages were not served from Varnish cache. Most of the time this was caused by flushes. Some needed and some not on purpose.
To get an easier grip on this I added the feature to log flushes, with the option to enable it from the Magento configuration (disabled by default).

Please merge my changes into the core devel branch.

Kind regards,
Jeroen Vermeulen.