Fix controller so that it not crashes when connection is closed

The controller has been crashing when the connection was dropped while
sending a message.
(Motivated by #201).
Add information on running LINC on Mininet with remote controllers

Now LINC can connect to a remote controller through the Mininet.
The controller definition is taken from command line options for mn.
(Motivated by #201).
Merge pull request #206 from FlowForwarding/issue201

LINC on Mininet with remote controllers