Fix balancing issue chaimleib/intervaltree#17

Well, this was the place where I got literally angry at you for, well, failing to implement the core data structure and having had to implement that logic myself I decided to express my disconcern by mentioning that my changes are not just minor tuneups any more.

In reality I don't care much whether the copyright message is one or two lines, of course. Also, if you want to interpret the MIT license as a requirement to keep the copyright note in the source code verbatim (which I personally do not thing is the case), then it was violated at the moment I added "et al" there first.