Verify that Psych is actually being used on 1.9.x
Add the ability to turn off the "safe load" warnings.

These warnings are useful, but very verbose for things like a large suite
of unit tests where you want the loads to be safe by default.
Support deserializing HashWithIndifferentAccess

We use serialized columns (via ActiveRecord::Base) heavily in our Rails app,
and most of them are hashes of the HashWithIndifferentAccess variety. Rather
than auditing everywhere a deserialized Hash is used and ensuring that it's
with_indifferent_access, the short term solution is to just have safe_yaml
support it natively.

(This only works with Syck, and is not that clean. A more complete whitelisting
arrangement would be better.)