Add a main_thread method to allow for serializing some invocations on the main thread

This wlil help different worker threads perform client acknowledgements or any
other STOMP client work, which should be done in the main thread to ensure that
we're not stomping on other messages

examples/simple.rb

@@ -21,7 +21,9 @@ class Worker < Stapfen::Worker
   end
 
   consume 'jms.topic.foo', {:ack => 'client'} do |message|
-    client.acknowledge(message)
+    main_thread do
+      client.acknowledge(message)
+    end
   end
 end
 

lib/stapfen/worker.rb

@@ -29,9 +29,14 @@ module Stapfen
 
     def initialize
       @pool = []
+      @workqueue = Queue.new
       handle_signals!
     end
 
+    def main_thread(&block)
+      @workqueue << block
+    end
+
     def run
       @client = Stomp::Client.new(self.class.configuration)
 
@@ -49,6 +54,16 @@ module Stapfen
         # close the connection, and an infinite Client#join call
         while client.open? do
           client.join(1)
+
+          until @workqueue.empty?
+            block = @workqueue.pop
+            begin
+              block.call
+            rescue => e
+              puts "Exception! #{e}"
+            end
+          end
+
           @pool = @pool.select { |t| t.alive? }
         end
       rescue Interrupt