Add libargon2 to github.com/winlibs and to the php-sdk

Hi!

We have support for password hashing via Argon2 in master now[1], which
relies on libargon2[2], so I suggest to add at least the relevant parts
of this library to <https://github.com/winlibs> and to the php-sdk ASAP,
so that the new feature can be worked with and tested under Windows.

Building Argon2Ref.lib in Visual Studio 2015 is trivial, as there are
project files available.  I'm not sure if it would be desired to have
makefiles, though.

[1] <https://github.com/php/php-src/commit/1305fdaa>
[2] <https://github.com/P-H-C/phc-winner-argon2>

-- 
Christoph M. Becker