Thread safety 

I think that a serious project must be thread safe.  I offer to add it to rmodbus-2 and this code will be working a perfect:

``` ruby
   sl1 = ModBus::TCPClient.new("192.168.1.1").with_slave(1)
   sl2 = ModBus::TCPClient.new("192.168.1.2").with_slave(2)

   Thread.new do
      loop { puts sl1.holding_registers[0..10] }
   end

   loop { puts sl2.holding_registers[0..10] }
```
