Give workers numbers to use in logging.
Make the logging format configurable, and have the default be machine-readable (JSON).
Move retry logic from Job.work into the work loop. This is cleaner and makes logging easier, too.
Revert "Give workers numbers to use in logging."

This reverts commit 0b847828e6dba899b70b11ce9ba96960143144ab.

Just not worth the hassle.

Conflicts:
	spec/unit/worker_spec.rb
Make it easy for jobs to do their own logging.
Oops, be more rigorous in what events we accept from Job.work.
A bunch of logging cleanup.
Add a doc on logging.
Add a migration system.
Make 100 the default priority.
Tweak a couple examples of job priorities in the README.
Let Que.migrate! work without a version.
Have migrate! take a hash with a version. It's a clearer API.
Generate Rails migrations that use the Que.migrate!(:version => 2) syntax.
Try to avoid killing threads, even in specs.
Discuss migrations in the advanced setup doc.
Add links to specific docs on installation outside of Rails and without ActiveRecord.
Write a bit about schema migrations in the changelog.