ESP8266Server class

Currently the usage of ESP8622Client is hard  when runnig as server because the ESP8622Client writes to the default channel if the channel id is not specified in the constructor. 

The new ESP8622Server class returns a ESP8622Client instance with the correct channel id set when data is available.

You probably had something similar already in mind. Please let me know if you want me to change something.
Use [ESP8266_MAX_CONNECTIONS](https://github.com/Diaoul/arduino-ESP8266/blob/master/ESP8266.h#L9) here