serial_conn + serial_thread new serial handling

Threading for extra thread to read from serial interface.
Write function amended with sleeping 2 seconds after writing.

Watch also Issue #15.

Signed-off-by: kroop <kroop@dresearch-fe.de>
connection.py flow control off and avoid dub conns

Flow control is disabled for every serial connection.
Connection class avoids opening more than one serial connections for one
serial port.

Watch also github Issue #15.

Signed-off-by: kroop <kroop@dresearch-fe.de>