Problem: it's painful to make new .gitignore by hand

Solution: generate this first time

Fixes #45
Problem: need way to specify model script

Solution: add 'script' property; if this is set then generate
appropriate gsl line. Note: we set -topdir:.. so the script works
from within src. (E.g. Java generates into ../main not into src).

Fixes #42
Merge pull request #52 from hintjens/master

Problem: it's painful to make new .gitignore by hand