[TAV-397] Add max_attempts method to the worker.

This is set to 1 so that each run is only tried once. Setting this within the
worker itself ensures that it will only be run once even if there are other jobs
within the main app that can be re-tried multiple times.
[TAV-399] Add generator to install delayed_job script.

This isn't picked up from within the gem when Player is installed in another
rails app so need to copy it manually like this.