Allow multiple applications use resque-scheduler on one redis

At the moment, if you start two different applications with different resque-scheduler configurations using one redis, only one will ever work as the master/slave lock key is the same.

This pull request allows the key to be prefixed, meaning multiple schedulers can be started for multiple different apps at the same time, thus:

    RESQUE_SCHEDULER_MASTER_LOCK_PREFIX=MyApp: rake resque:scheduler
    RESQUE_SCHEDULER_MASTER_LOCK_PREFIX=HerApp: rake resque:scheduler