add update-submodules.sh script

Reviewer: @jnealtowns

This script automates the process of updating every submodule, and it includes 
a nice commit message like:

    update indigo

    * submodules/indigo 1a2b943...216a076 (2):
     > Merge into master from pull request #158: OFConnectionManager: add notifications for barriers (https://github.com/floodlight/indigo/pull/158)
     > Merge into master from pull request #157: Add generation ID to the connection ID, and pause reading messages during long-running tasks (https://github.com/floodlight/indigo/pull/157)