Additions and fixes to 2015/endoh3

Fixed Makefile so that make ; ./prog will work.

He provided additional source files that do different things. See the
README.md for details and how to run (try section). He said that there
should be a tarball with other source files but I don't see it and he
doesn't have a download. I'll ask him for it later.

Updated .gitignore.

2015/endoh3/Makefile

@@ -131,7 +131,7 @@ all: data ${TARGET}
 	supernova deep_magic magic charon pluto
 
 ${PROG}: ${PROG}.c
-	${CC} ${CFLAGS} $< -o $@ ${LIBS}
+	${CC} ${CFLAGS} $< -o $@ ${LIBS} ${BACK_TO}
 
 # alternative executable
 #