change gitignore, host_vars and hosts should be checked in

this is important so the playbook works properly
check in host_vars/10.0.0.1 and vagrant hosts
delete example.io file
only install github-sentry if github credentials are provided
proper check if github_* vars are set
install missing git-core dependency
make sure that nginx is running
migrate the sentry database automatically
remove obsolete entry from README
Merge pull request #1 from nicolai86/master

make the ride less bumpy