Added options -destdir and -ext

New options for ocamlbuild integration:
  -ext c          forces tool go generate source files with .c extension
  -ext cpp        will generate .cpp files (default)
  -destdir <path> sets output directory where to put generated files;
                  "." is default value
Fixed help. Option -list was added.

Option -list prints class names which are used in the input file.