Fix timeouts on Unix platform

[SteveEasley]

Replaces all time.clock() calls with time.time(). On the the Unix platform time.clock() is not wall-time like it is on Windows. It results in timeouts that last orders of magnitude longer than expected on the Ubunutu 16.04 system I am working on. For instance this code was talking over an hour to finish:

xbee = ZigBeeDevice("/dev/ttyUSB0", 9600)
xbee.open()

xnet = xbee.get_network()
xnet.set_discovery_timeout(3.2)

xnet.start_discovery_process()
while xnet.is_discovery_running():
    time.sleep(0.5)

# >>> would take over an hour or more to get here

for device in xnet.get_devices():
    print(device)

print('done')

Also time.clock() is depreciated.

My PR is very untested beyond the code I provided above as I have just begun using the lib. So more testing is needed.