changed folder structure corresponding to maven guidlines

src/main/resources/com/ardublock/block/ardublock.xml

src/main/webapp/reference/404.html

src/main/webapp/reference/loop.html

src/main/webapp/reference/loop.jpg

src/main/webapp/reference/loop.png

src/main/webapp/reference/program_comment.html

src/main/webapp/reference/program_comment.png