Support for POSIX tcflush(3)

_ruby-serialport_ lacks the capability to discard data transmitted and/or received through the serial effectively, function that is already provided by the tcflush(3) function from termios on _POSIX_ compliant system.

This pull add support to flush the data received and/or transmitted through the following method.

1.   SerialPort#flush_input()
2.   SerialPort#flush_output()
3.   SerialPort#flush   # both input and output.