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)
I tried a few things, but the problem is git often detaches the submodule from any branch. Without knowing what the branch is we can't get an upstream to pull from (and the user would have to configure the upstream anyway, which isn't done by git-submodule).

Instead, I added a `-n` option that disables the checkout, reset, and pull steps. When it's necessary you can do those steps yourself for any remote branch you want, and then use this script to generate the commit message.

I also added a `-b` option to change the remote branch which might be good enough for most cases.

I added the ability to get the submodule paths from the command line, so you can selectively update submodules with different combinations of `-n` and `-b`.