Added the name of a CsvReporter to CsvReporter.Builder

I've described the problem in issue 469 (https://github.com/codahale/metrics/issues/469) - this is my suggestion to how the issue could be resolved:

The reporter name passed to the super class (AbstractPollingReporter) was hard-coded to "csv-reporter" meaning that all CsvReporters would get the same thread pool, and couldn't be started and stopped independently.

Adding the name to the Builder makes it possible to give a CsvReporter a unique name and thereby a new thread pool.