Refactor WebSocket support into separate sync/async implementations

[dolfies]

Addresses #202 WebSocket requests

• Separates async support into AsyncWebSocket
• Removes Session.ws_connect() and allows initialization via WebSocket() constructor directly
  • This is because WebSockets need their own curl handle, as making a request will break the CurlOpt.CONNECT_ONLY value, so the session becomes useless if a WebSocket is created from it
  • This is not an issue with AsyncSession.connect() as it already maintains multiple curl handles
• Changes AsyncWebSocket API to more closely match aiohttp
  • Removes sync-style callback handlers
  • Adds send/recv_str/json helper methods
• Adds on_data callback to WebSocket
• Adds autoclose and skip_utf8_validation parameters, and close_code and close_reason attributes
• Adds iteration support

TODO:

• Test and refine
• Figure out why concurrent curl_ws_send() calls break unfortunately seems unsupported :(
• Verify which curl opts are useful for WebSocket connections
• Figure out how autoclose should behave with errors
• Implement reconnection logic to run_forever()
• Possibly wait for server to close connection before closing

[yifeikong]

Removes Session.ws_connect() and allows initialization via WebSocket() constructor directly

aiohttp uses ws_connect.

[dolfies]

aiohttp uses ws_connect.

AsyncSession still has ws_connect().
I removed it from sync Session since websocket-client uses the constructor and the curl handle can't be shared anyway.