make it possible to inject a custom worker for QC's rake tasks.

In every project I'm using QueueClassic we end up writing
our own Worker. This is also encouraged in the README.

The process is very smooth but one drawback is that you can't
continue to use the rake tasks, which ship with QC. Or at least
not `rake qc:work` as this would always use the default worker.

I think we should encourage custom workers and make it trivial
to inject them into QC's rake tasks.

lib/queue_classic.rb

@@ -70,6 +70,14 @@ def self.default_conn_adapter=(conn)
     @conn_adapter = conn
   end
 
+  def self.default_worker_class
+    @worker_class ||= QC::Worker
+  end
+
+  def self.default_worker_class=(worker_class)
+    @worker_class = worker_class
+  end
+
   def self.log_yield(data)
     begin
       t0 = Time.now

lib/queue_classic/tasks.rb

@@ -8,7 +8,7 @@
 namespace :qc do
   desc "Start a new worker for the (default or $QUEUE) queue"
   task :work  => :environment do
-    @worker = QC::Worker.new
+    @worker = QC.default_worker_class.new
 
     trap('INT') do
       $stderr.puts("Received INT. Shutting down.")

test/worker_test.rb

@@ -27,6 +27,18 @@ def handle_failure(job,e)
 end
 
 class WorkerTest < QCTest
+  def test_qc_worker_is_the_default_worker
+    assert_equal QC::Worker, QC.default_worker_class
+  end
+
+  def test_default_worker_can_be_changed
+    original_worker = QC.default_worker_class
+    QC.default_worker_class = TestWorker
+
+    assert_equal TestWorker, QC.default_worker_class
+  ensure
+    QC.default_worker_class = original_worker
+  end
 
   def test_work
     QC.enqueue("TestObject.no_args")