Fix issue 38, 44 and 185

These changes are for https://github.com/mininet/mininet/issues/185 . 
String equivalents of dumpNetConections and dumpNodeConnections have been added to util.py. It is now possible to retrieve as a list of strings, the connections for the hosts or entire network.

The change of https://github.com/mininet/mininet/issues/38 is in mininet/mininet/net.py

The regression tests for issue https://github.com/mininet/mininet/issues/44 are in mininet/mininet/test/test_dpidassignment.py