Fix YAML/SafeYAML mix-up with Guard.

Guard and SafeYAML do not play well together (See
https://github.com/dtao/safe_yaml/issues/10#issuecomment-13039602).
We can avoid patching YAML but just requiring 'safe_yaml/load' and
then load our YAML data with SafeYAML.load instead of YAML.load.
Configure SafeYAML once instead of on every request.
Extend LiveReload grace period a bit.