Add a reporter for logging metrics to SLF4J Loggers

This pull request adds the ability to periodically report metrics through the SLF4J logging framework. It is modeled after the other polling reporters like CsvReporter and ConsoleReporter. It allows more flexibility to report metric information for things other than just dumping to the console or a flat csv file. It also supports specifying a Marker instance for filtering and processing by downstream appenders and filters in the bound logging package. (For example, a special log file with just metrics results.)