Add a mechanism to switch out loggers

[stevegrunwell]

Version 0.1.0 of the plugin is launching with only the DatabaseLogger Logger, but the plugin is designed to be flexible enough that people can write their own custom loggers. One of the major items for version 0.2.0 will be refactoring get_logger() so users, either through a filter, option, or some other means can say "no, don't even touch the DatabaseLogger class, use MyCustomLogger instead.