text: Title: can not add workers in API.run
  Body: hi,
  - My run:
  ```python
  api = responder.API()
  ......
  api.run(address='192.168.1.56', port=1005, workers=2)
  ```
  - Get error
  ```python
  Traceback (most recent call last):
    File "E:/WorkDir/WorkDir/Taobao/ParseTklByResponder/app/parseTKL.py", line 174, in <module>
      main()
    File "E:/WorkDir/WorkDir/Taobao/ParseTklByResponder/app/parseTKL.py", line 169, in main
      api.run(address='192.168.1.56', port=1005, workers=2)
    File "C:\Users\admin\.virtualenvs\ParseTklByResponder-fJ87HmF5\lib\site-packages\responder\api.py", line 687, in run
      self.serve(**kwargs)
    File "C:\Users\admin\.virtualenvs\ParseTklByResponder-fJ87HmF5\lib\site-packages\responder\api.py", line 682, in serve
      spawn()
    File "C:\Users\admin\.virtualenvs\ParseTklByResponder-fJ87HmF5\lib\site-packages\responder\api.py", line 680, in spawn
      uvicorn.run(self, host=address, port=port, debug=debug, **options)
    File "C:\Users\admin\.virtualenvs\ParseTklByResponder-fJ87HmF5\lib\site-packages\uvicorn\main.py", line 274, in run
      supervisor.run(server.run, sockets=[socket])
    File "C:\Users\admin\.virtualenvs\ParseTklByResponder-fJ87HmF5\lib\site-packages\uvicorn\supervisors\multiprocess.py", line 33, in run
      process.start()
    File "C:\Users\admin\AppData\Local\Programs\Python\Python36-32\Lib\multiprocessing\process.py", line 105, in start
      self._popen = self._Popen(self)
    File "C:\Users\admin\AppData\Local\Programs\Python\Python36-32\Lib\multiprocessing\context.py", line 223, in _Popen
      return _default_context.get_context().Process._Popen(process_obj)
    File "C:\Users\admin\AppData\Local\Programs\Python\Python36-32\Lib\multiprocessing\context.py", line 322, in _Popen
      return Popen(process_obj)
    File "C:\Users\admin\AppData\Local\Programs\Python\Python36-32\Lib\multiprocessing\popen_spawn_win32.py", line 65, in __init__
      reduction.dump(process_obj, to_child)
    File "C:\Users\admin\AppData\Local\Programs\Python\Python36-32\Lib\multiprocessing\reduction.py", line 60, in dump
      ForkingPickler(file, protocol).dump(obj)
  TypeError: can't pickle _thread.lock objects
  ```
  
  python 3.6.5
  windows 7
  responder 1.3.0
  
  I am not sure if API.run supports adding workers. 
  what should I do?
  Thanks a lot !