Adds the beanstalkd/beanstalkd-metrics.rb plugin

All it does is take the metrics provided by beanstalkd and push them to
graphite. There's other plugins in beanstalk/ (no d at the end) but those use
the outdated 'beanstalk' ruby gem.
Merge pull request #459 from paasta/beanstalkd-metrics

Adds the beanstalkd/beanstalkd-metrics.rb plugin