Tree support for JSON patch operations

[jgustie]

The JSON pointer implementation in 2.3 is a big part of an RFC 6902 patch implementation. I was hoping maybe to also see add(JsonPointer, JsonNode) and remove(JsonPointer) methods on JsonNode. All of the operations in the spec can be built on top of these in a higher level JSON patch implementation.

It looks like the mutating operations are usually further down in the array and object node implementations, but I thought maybe it made sense for the patch operations that take a JsonPointer to be on JsonNode to make them easier to call.

[cowtowncoder]

The main design criteria here is that JsonNode only contains read methods, not mutation methods. This allows potential immutable JsonNode implementations.