housekeeping: make AGPL (affero gpl v3) everywhere

Some basic house keeping, apply a single license consistently
everywhere, as well as actually include a license file.

I got this from:
* http://www.gnu.org/licenses/agpl-3.0.txt
* http://choosealicense.com/licenses/agpl-3.0/
tweak COPYRIGHT assertion

Discussed with core team, hopefully following recommendations found
under "maintaining centralized copyright notices" found:

* http://softwarefreedom.org/resources/2012/ManagingCopyrightInformation.html
Merge pull request #5 from nightscout/dev

GPL