Make bin/pegjs work when called via a symlink

Similar issue exists on Windows too (they have symlinks since Vista), but I
could not find how to dereference symlinks from batch files, so I did not fix
it. I guess this does not matter much given how little the symlinks are used in
the Windows world.

Closes pegjs#1.

bin/pegjs

@@ -1,4 +1,10 @@
 #!/bin/sh
 
-DIR=`dirname "$0"`
+if [ -L "$0" ]; then
+  THIS_FILE=`readlink "$0"`
+else
+  THIS_FILE=$0
+fi
+DIR=`dirname "$THIS_FILE"`
+
 java -jar "$DIR/../vendor/rhino/js.jar" "$DIR/pegjs-main.js" "$DIR" "$@"