Add AGPLv3+ license (closes #7)

If you want to release your code as free software under the terms of the GNU Affero General Public License version 3, or later, here is how to do so.

Issue #7 mentions the link where to find more information if you'd like to choose another license.  

I recommend [AGPLv3+](https://gnu.org/licenses/agpl) for two reasons: 

1.  it covers the case of interception and code injection by a third party (MITM), which is one of the attack vectors of surveillance systems using Javascript.  In practice, it legally covers that the source code you see on the server, is the code you run on your client machine;
2.  it forces modifications that are to be distributed (e.g., over the network) to be released under the same terms, therefore making it both illegal and difficult to reuse the code to perform an attack on users' privacy.

Both reasons are more symbolic than effective, but they stand along with the idea of fighting massive surveillance.  See the [GNU LibreJS](https://gnu.org/s/librejs) project for more information about the [Javascript Trap](https://gnu.org/philosophy/javascript-trap.html).