Added a json_size function that returns for the 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())
Merge branch 'issue_1284' of https://github.com/nezihyigitbasi/presto into issue_1284

Conflicts:
	presto-main/src/test/java/com/facebook/presto/operator/scalar/TestJsonFunctions.java