json_size implementation

This pull request fixes #1284. One thing that needs to be done is to update the documentation for this new function.

This pull request implements a json_size function that returns for a JSON node "the number of child nodes this node contains: for Array nodes, number of child elements, for Object nodes, number of fields, and for all other nodes 0." that's extracted from the given JSON string using the given JSONPath expression (see
com.fasterxml.jackson.databind.JsonNode#size())