Introduces YAML safe loading and dumping

For backward compatibility, loading or dumping unsafe YAML contents only produce a DeprecationWarning if the function has not been explicitly called with a safe argument.

Loading or dumping with safe=False causes behavior identical to releases predating this one.
Loading or dumping with safe=True will ensure that only safe contents are allowed. Contents deemed unsafe by the pyyaml module will raise a SerializationSafetyError.