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 
In some log messages, I see `'{user_id}'` (in quotes), but here I see `{user}` (no quotes). Should we be consistent and put quotes around the interpolated string, or not use the quotes at all?