Add script to lint javascript files staged in git

This can be used in a git pre-commit hook:

```shell
cat > .git/hooks/pre-commit <<EOF
!/bin/sh
./script/git-pre-commit-jshint.sh
EOF
chmod +x .git/hooks/pre-commit
```
Also fix a couple of JSHint violations. I've ignored violations fixed in another current PR: https://github.com/moneyadviceservice/dough/pull/183