Load safe_yaml without patching YAML.load
Don't set safe_yaml global options

This way the users can decide for themselves, and tolk can pass
the desired options to each SafeYAML.load call.
Extract YAML handling to a Tolk::YAML wrapper
Move ya2yaml checking to Tolk::YAML.dump
Merge pull request #84 from grk/safe-yaml-load-fix

Load safe_yaml without patching YAML.load