i18n build only for a subpackage.

It should be possible to build the i18n files only for a subpackge for example `opengever.core/opengever/dossier`. Because when using the i18n-build script, it allways ends with an useless diff.

@lukasgraf it would be nice when the `i18n-build` would take the subpacke as first parameter: 
`bin/i18n-build dossier`