PNG support

It allows you to decode a PNG file (including alpha channel).

Also modifies display.blit() to accept image data with an alpha channel. Introduces an RGB5551 format for RGB data with a single alpha bit.

.gitmodules

@@ -10,3 +10,6 @@
 [submodule "lib/crypto/SHA256"]
 	path = lib/crypto/SHA256
 	url = https://github.com/ilvn/SHA256
+[submodule "lib/lodepng/lodepng"]
+	path = lib/lodepng/lodepng
+	url = https://github.com/lvandeve/lodepng