Hit Selection Mode > Press and hold "s" for source node and click on the node you want > Press and hold "t" for target node and click on the node you want > Once the names of the nodes appear to the left under the header "+/- Edge", hit Add Edge.
I created GraphBin (http://graphbin.co) because my data structures and algorithms prof didn't want us to hand draw graphs for assignments. I didn't want to make them via paint either.
Maybe not a general purpose tree editor, but at least it's helped me out so far :)