Revises the licensing section

[hlapp]

Closes #8.

[hlapp]

I need to get this ready for teaching tomorrow. Any comments from anyone (esp. @dsoto, @elliewix, @raynamharris, @ErinBecker) before I merge this?

[elliewix]

• might want to define/link out to what ePHI is, and potentially include more examples, e.g. protected human subject data, proprietary data from another party, health information such as PHI or HIPAA information, or data that could threaten national security
• expand/link out to what DUA is

[hlapp]

Thanks @elliewix. I think I addressed both of your comments. As for DUAs, I really don't think this is a good place to promote these, but I added information as to why, and some examples people might encounter to the instructor notes.

[elliewix]

http://www.openstreetmap.org/copyright might be another interesting use case to highlight

[hlapp]

Indeed. Added to the instructor notes.