[gt-users] skeleton code generator script

[Gordon Gremme]

Dear Giorgio,

many thanks for the codegen script, I think it is quite useful.
I just pulled your branch into the master. I made some small
corrections, I hope you agree with them.
Most importantly, I renamed the script from codegen.rb to codegen,
because high level scripts should not have an ending which reflects
the implementation language (similar to the scripts src_check and
src_clean).

Gordon