Concurrent processing of jobs

More than 1 job can now be run simultaneously, if rateLimit !== null.

Provides job processing statistics

New:

• Emits stats events regularly, based on the statsInterval option (as provided at instantiation)
• Event provides the nbJobsPerSecond statistic

Simple implementation: no concurrency

Features:

• can run up to 1 job at a time
• events: done, drain, error
• ability to start and pause the queue
• optional logging/tracing (debug)
• automated tests with mocha