* Add new command-line option:   -f <filename>   Read the flow(s) to install from a file   File format: one line per flow starting with <flow-id>   Thus, a single script can install and maintain multiple flows.
* Add new command-line option:
  -f <filename>   Read the flow(s) to install from a file
  File format: one line per flow starting with <flow-id>
  Thus, a single script can install and maintain multiple flows.

* NOTE: Change the behavior: now the computed shortest path is printed
  ONLY when there is a change, instead of periodic printing once
  every second.

* If there is a failure in computing the shortest path, print
  the path endpoints.

opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open