Sanitize metric names to avoid API errors

Hi
I encountered API errors when exporting mbeans containing spaces or other non-alphanumeric characters, so I added a method to clean up metric names, based on a regexp copied from https://github.com/librato/librato-java/blob/master/src/main/java/com/librato/metrics/APIUtil.java 
Check test/test-publisher.rb for metric names examples
Feel free to improve it :)