Improve WebSocket close implementation

[dolfies]

• Allow passing close code/message directly to close()
• Fix aclose() not accepting parameters
• Implement close code/message parsing and provide to on_close handler
  • Might make sense to put this in recv() and return a class instance containing parsed data instead of raw bytes, similar to aiohttp
• Add close code enum and expose WebSocket enums
• Close socket on protocol error (as per spec)
  • Should probably check for other protocol errors in the future and raise accordingly
  • Clients are supposed to reply with a close message whenever they receive a close frame no matter what (as per spec), so consider doing that as well (though not entirely necessary, can be left up to user)

[perklet]

LGTM