added Apache 2.0 license

[filipw]

The absence of license is extremely problematic in many (most? all?) jurisdictions. If you would like to grant users the freedom to use, modify, and distribute the project, you need to attach an explicit permissive open-source license to the project.

I assume this is just an oversight - since I can see pretty much all of your other Github projects use Apache 2.0 license. I added it here too.

Thanks!