Fixed automatic file naming. Added -e and -d command line options
to force en- and decoding. Added detection of invalid PNG input
when decoding. Added a script for generating README.md with the
latest usage instructions on "make". It takes the text from
README.in and automatically includes the usage instructions
compiled into s2png. Cleaned up rc4.c. Improved the formatting of
s2png.c (now it's 80 cols) and changed some types.