Merge pull request #153 from fge/master

Looks good, will merge -- ObjectNode and ArrayNode: make _children final
ArrayNode: make it final

Take the opportunity to get rid of "non-final-only" methods along the way.
Make ObjectNode final

This also means a test had to be removed, which tested whether JsonNodeFactory's
.objectNode() could be overriden.

However, other methods of JsonNodeFactory, overridable, also return final
classes (.binaryNode(), .textNode() etc).