Allow configurable index/removal jobs

The above change-set allows the default index/removal jobs for resque and sidekiq to be overridden via the following syntax:

    Sunspot::Queue.configure do |c|
      c.resque_index_job = MyCustomIndexJob
      c.resque_removal_job = MyCustomRemovalJob
      c.sidekiq_index_job = MySidekiqIndexJob
      c.sidekiq_removal_job = MySidekidRemovalJob
    end

Or without a block accessing the configuration object directly:

    Sunspot::Queue.configuration.resque_index_job = MyCustomIndexJob

There are default jobs provided based on what was being used.