print -> info; end the horror of print vs. print()

Although we could use print() from __future__, this messes
up scripts which use examples as modules.

The simple, if not nicest for 2.7, solution is to use
info(), output() and other mininet.log functions. The disadvantage
is that we may have to adjust things if we change info() to
add automatic newlines, but we can burn that bridge in Mininet
3.x.

examples/nat.py

@@ -6,7 +6,7 @@ Example to create a Mininet topology and connect it to the internet via NAT
 
 
 from mininet.cli import CLI
-from mininet.log import lg
+from mininet.log import lg, info
 from mininet.topolib import TreeNet
 
 
@@ -16,8 +16,8 @@ if __name__ == '__main__':
     # Add NAT connectivity
     net.addNAT().configDefault()
     net.start()
-    print( "*** Hosts are running and should have internet connectivity" )
-    print( "*** Type 'exit' or control-D to shut down network" )
+    info( "*** Hosts are running and should have internet connectivity\n" )
+    info( "*** Type 'exit' or control-D to shut down network\n" )
     CLI( net )
     # Shut down NAT
     net.stop()