Add a check enforce "samples/" folder to be up-to-date

Created by: jmini

I think that with each PR, the samples/ needs to be completely updated. For the moment people focus only on their generators (which is fine), but this is wrong.

First reason: some CI Jobs are updating /samples with /bin/run-all-petstore and then run each generated project. So they are not executed against code that is checked in, but against code that is generated.

Second reason: for later analysis (understand what was broken when) it is good to have the changes in the same PR.

Third reason: even if you think that you have only updated one generator (say java) your change might have an undesired impact in a other generator. Having it explicitly is good. This is even worth when you change something in DefaultGenerator (see my mistake in #79 (closed))

Pre-requisit:

All generators needs to support the hide generation timestamp feature.
All examples in the samples/ project needs to have been generated once

The check should looks like this and could be added at the end of bin/run-all-petstore:

if ! `git diff-index --quiet HEAD -- `; then
    echo "There are uncommitted changes in working tree after execution of 'bin/run-all-petstore'"
    git status
    echo "Please commit changes"
    exit 1
else
    echo "Git working tree is clean"
fi