Recursive Find

Find an item by name recursively.
On these to lines, I would suggest to enforce the "top-levelness" of the members as well:
````
EXPECT_EQ(doc.FindMemberRecursive("first1"), doc.FindMember("first1");
EXPECT_EQ(doc.FindMemberRecursive("first2"), doc.FindMember("first2");
````
Eventually, you could add a "duplicate" key to different levels of the hierarchy in the JSON string to check the _breadth-first search_ ordering.