FTP and partial Telnet support

This pull request adds FTP and Telnet support (Telnet is not fully implemented yet). FTP has been tested on Linux and Windows 8.1. Messages written with DEBUG_MESSAGE are also flushed after sending, because this seems to help a bit with debugging whenever firmware hangs occur.

In a future release, I will try to reduce the Webserver's memory usage by another 1-2K, which I think can be achieved by reducing the size of clientMessage and by storing any upload data in uploadBuffer.