Added the Apache License the GitHub way

[nrogoff]

I noticed that the License Url for the extension was Apache v2. So added it. This way it also shows in the GitHub interface too.

[sjkp]

I'm not sure that the apache license is even the right one. I think I just put it in the original nuget, because I had to put something there. I think Mozilla Public License Version 2.0 would be better, but I know close to nothing about software licenses.

[nrogoff]

OK. Just reject this PR and you can add the license you want. I add the procedure to my blog, so I could remember! https://blog.nicholasrogoff.com/2017/01/12/how-to-add-open-source-license-to-your-github-repository/ and you can here is the site to help you choose which one you want http://choosealicense.com/