Create and retrieve bucket loggers using arbitrary names

This adds the feature of being able to create and retrieve bucket loggers using an arbitrary name instead of a learner id. Bucket loggers defined by a name will not have any learner associated with them.

The idea is that it is up to the application to ensure that the named loggers are unique (if that is desired by the application). In the case of Geniverse, we can use the user's user names, which are unique for each user.

@psndcsrv Can you take a look at this and see if you see any problems?