Update readme examples for Python 3

[Stevoisiak]

The code examples provided in README.md for Using MultiPing are not compatible with Python 3, as they use Python 2 style print statements.

The receive() function returns a tuple containing a results dictionary (addresses and response times) as well as a list of addresses that did not respond in time. The results may be processed like this:

...

for addr, rtt in responses.items():
    print "%s responded in %f seconds" % (addr, rtt)