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 
Nitpick, but I've seen us write these kinds of lines like this in the past:
```python
u"Verification for user '{user_id}' approved by '{reviewer}'.".format(
    user_id=self.user, reviewer=user_id
)
```
I think it makes it a little clearer which string the `format` is being applied to. Like I said, this is a nitpick, but I wonder if it would be good to try to stick to the conventions we've followed in the past.