Adding BSD-style license

As per NPCH's recommendations, to address #22.  @jiffyclub can you please check?
If it's Software Carpentry, it needs to be a legal entity (e.g. Software Carpentry, LLC) and we should have a contribution process that means that any contributors to this codebase sign over copyright to Software Carpentry (or some "holding" organisation) via a Contributor Licensing Agreement (CLA). This is the model that is popular in many of the larger open source projects that have a need to control contributions and IP and perhaps relicense in the future. See http://oss-watch.ac.uk/resources/cla for a good guide.

The alternative is as Greg has currently implemented it: the overall codebase is licensed across many authors. Each contributor should be acknowledged by name/organisation, ideally in the source files they contributed to. This is more "community-oriented" but does mean that if we want as a group to make changes later to the license, we must get in touch with all the authors it affects.