Add explicit license file and source code headers

[raphlinus]

Including a copy of the license, and explicit headers in source files, is consistent with best open-source practice. This is important partly because I plan to check in a dependency with a different license.

[raphlinus]

See Google's opensource documentation for reference.

Note, I've left the copyright attribution as I found it. For multiple authors, the most technically correct approach is to change it to "The Phil Authors" and have a separate AUTHORS file that lists them individually. If you're open to that, I'm more than happy to tweak this PR.

[raphlinus]

Thanks for doing this in master.