Adding a number of logging messages for verification process.

Simply adding a number of logging messages in at the info / warn / error level.

* Any exceptions raised will be logged as well
* Info level messages to catch entry points of verification flow
* Info level messages for verification approve / deny
* Already good info level messages for purchase callbacks in place.

@wedaly @rlucioni @dianakhuang 
Another nitpick, but I see that this string isn't marked as unicode despite the other log messages you've added being marked as such. It'll be converted to unicode when by `ugettext`, but the logged error message itself won't be unicode. Maybe it'd be a good idea to be consistent and mark it as unicode?