unique jobs

[scones]

short description:

clone this funtionality: https://github.com/resque/resque-loner

(a job with the same payload will only be added once)

This is particularly mighty in combination with delayed jobs.

[merlindorin]

@scones I really like the plugin design (was thinking about that for the feature "retry").

We have everything we need to do that in php-resque. We just need a register method somewhere. Anyone motivated for such implementation?

[scones]

depends on #82, obviously