Move the responsibility of safe loading to Node

[michaelherold]

This change moves the responsibility of safely loading a YAML tree from an internal detail of Psych.safe_load into Psych::Nodes::Node.

The advantage of this is that you can now more easily safely load trees that you're working on without replicating the safety constraints of Psych.safe_load. The delegation of responsibilities also now matches for safely and unsafely loading a tree.

I opted for two choices to make the diff smaller:

1. I left all of the documentation on Psych.safe_load instead of moving it to Psych::Nodes::Node#to_safe_ruby. The main reason I chose to do so is that Psych.safe_load is the most likely entry point into using Psych for most people, so having the documentation on that method makes sense.
2. I delegated responsibility for the default arguments to Psych::Nodes::Node rather than duplicating them. The reason I chose to do this was to reduce the chance of two lists of arguments becoming unsynchronized. Additionally, since the behavior actually belongs to Psych::Nodes::Node now, it's the only place that can have the defaults set for all use cases.