Add Apache 2 license file - fix #14

I did not pick the license myself, of course, I just looked at http://martiansoftware.com/nailgun/license.html, then I created LICENSE.txt and picked the GitHub template, and finally I copied the copyright notice (the first line) from the website.
Merge pull request #31 from Blaisorblade/patch-2

Add Apache 2 license file - fix #14