Add Message#cmd_sym as a convenience method

spec/btc_wire_proto_spec.rb

@@ -235,6 +235,11 @@ describe ::BtcWireProto do
         m.payload.selection.should == "version"
       end
       
+      it "should have a :version command symbol" do
+        m = Message::new(:header => {:command => 'version'})
+        m.cmd_sym.should == :version      
+      end
+      
       it "should parse binary data correctly" do
         m = Message::read(binary(%w{
           F9 BE B4 D9 76 65 72 73 69 6F 6E 00 00 00 00 00 55 00 00
@@ -265,6 +270,11 @@ describe ::BtcWireProto do
         m = Message.new(:header => {:command => "verack"})
         m.payload.selection.should == "null"
       end
+      
+      it "should have a :verack command symbol" do
+        m = Message.new(:header => {:command => "verack"})
+        m.cmd_sym.should == :verack
+      end
 
       it "should parse the binary data correctly" do
         m = Message::read(binary(%w{