gitignore: add build_package folder.
build_desura.sh: add missing exit status.
install_deps.sh: note pack_deb/rpm options if we are on debian/redhat
managers: add SingletonHolder template
managers: LanguageManager should be created without arguments
managers: use SingletonHolder for global services
managers: use reference semantic not pointer
README: mention ./build_desura.sh help
Merge pull request #493 from matthiaskrgr/stuff

Stuff
Merge pull request #490 from karolherbst/bug190

manager: change global objects to singletons