Add a new filter that matches against a logger name

This patch provides a new logging filter, similar to the existing string matcher, that matches against a logger name. It provides options for case sensitivity and exact matching, as well as the standard acceptOnMatch and stringToMatch parameters. Unit tests and documentation are included. 

My use case for this is included in the documentation; I've found it useful to log events from specific sources to their own logfiles, so I can have a standalone log for payment gateways, SMS sending, etc. Sometimes this is for ease of debugging; sometimes it makes integration with other tools like Munin easier.