Add script to generate the official-images library manifest file

Usage:

```console
$ ./generate-stackbrew-library.sh > /path/to/official-images/library/maven
$ cd /path/to/official-images
$ # make a PR out of the changes \o/
```

Example output:
```console
$ ./generate-stackbrew-library.sh
# maintainer: Carlos Sanchez <carlos@apache.org> (@carlossg)

3.2.5-jdk-6: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-6
3.2-jdk-6: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-6
3-jdk-6: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-6

3.2.5-jdk-6-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-6/onbuild
3.2-jdk-6-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-6/onbuild
3-jdk-6-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-6/onbuild

3.2.5-jdk-7: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7
3.2.5: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7
3.2-jdk-7: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7
3.2: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7
3-jdk-7: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7
3: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7
latest: git://github.com/carlossg/docker-maven@f7b5c6e868367cf05311a9883c0cda908edf808c jdk-7

3.2.5-jdk-7-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild
3.2.5-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild
3.2-jdk-7-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild
3.2-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild
3-jdk-7-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild
3-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild
onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-7/onbuild

3.2.5-jdk-8: git://github.com/carlossg/docker-maven@596e9014bba56dae2a0bdac69b4f5492bc95dad5 jdk-8
3.2-jdk-8: git://github.com/carlossg/docker-maven@596e9014bba56dae2a0bdac69b4f5492bc95dad5 jdk-8
3-jdk-8: git://github.com/carlossg/docker-maven@596e9014bba56dae2a0bdac69b4f5492bc95dad5 jdk-8

3.2.5-jdk-8-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-8/onbuild
3.2-jdk-8-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-8/onbuild
3-jdk-8-onbuild: git://github.com/carlossg/docker-maven@b022df671b603a9100ed9e75803ae32f753826a4 jdk-8/onbuild
```