Created Node::getLogicalBlock().

Here's a convenience method on Node (because it's a convenience method, I didn't bother to add it to NodeInterface) which will return the closest StatementBlockNode or TopNode, which as far as I can tell should cover just about any context which could be a code block (if, elseif, else, do/while, for, case, class, function, top level...did I miss any?)

From issue #15.