Breaking change: Legend.AddEntry -> use same argument order of ROOT

With reference to [this 
comment](https://github.com/rootpy/rootpy/commit/bbcb64a5f99e00bb73894de81acbaf49e789288b#commitcomment-3372263).

It seems really bad juju to me to change the argument order between rootpy and root. Ideally, one should be able 
to swap out plain-root classes for rootpy ones transparently.

Apologies to any existing code this breaks, but I think it might be worth it going forward.