remove() always returns undefined

[mfogel]

Hi @w8r, thanks for your work on splay-tree. Very helpful lib.

Per the docs in the README, remove() is intended to return a boolean. Per the docs in remove()'s docstring, it should return a node or null. However, it just returns undefined, irrespective of whether it removed anything or not from the tree.

Note that it appears this is addressed by an open PR: #5