Add 3 piece sets (mainly Merida & Alpha).

I was avoiding non-free piece sets, but since we're already using
the Alpha font (to export to PDF there's basically no libre
option), there's no point in not including the SVG pieces too.

And since we're at it, I added Merida too, since it's probably the
most popular set of them all. And after all, despite being non-free,
I think it's in the spirit of the authors' intentions to use them in
a free app like this one.

I also added Governor, another of sadsnake's sets, this one is
creative commons and has a very elegant design.

README.md

@@ -42,5 +42,6 @@ I can't promise anything though, since this is just a hobby project (and the goa
 ### Assets authors / licenses:
 - The piece set "cburnett" is a work of Colin M.L. Burnett and used under the CC-BY-SA 3.0 unported license (more info on the license.txt file in that directory).
 - The "california" piece set is a work of Jerry S. licensed under CC BY-NC-SA 4.0 (https://sites.google.com/view/jerrychess/home)
-- The piece sets "Cardinal", "Dubrovny", "Gioco", "Icpieces", "Maestro", "Staunty" and "Tatiana" are work of "sadsnake1", licensed under CC BY-NC-SA 4.0. And obtained from the lila (lichess) repository.
-- The font "Chess Alpha" is a work of Eric Bentzen and free for personal non commercial use. Full info in the documents in the "font" directory.
+- The piece sets "Cardinal", "Dubrovny", "Gioco", "Icpieces", "Maestro", "Staunty", "Governor" and "Tatiana" are work of "sadsnake1", licensed under CC BY-NC-SA 4.0. And obtained from the lila (lichess) repository.
+- The piece set and font "Chess Alpha" is a work of Eric Bentzen and free for personal non commercial use. Full info in the documents in the "font" directory.
+- The original Merida chess font is a work of Armando Hernandez Marroquin and distributed as 'freeware' and the shaded version used here and obtained from the lichess repository is a work of Felix Kling ("DeepKling" here on github).