no support for darwin (OS X)

[LucaFilipozzi]

Very interesting project. I very much appreciate your pure-python implementation of an ssh agent proxy. As I wish to make use of a variant of this project on an OS X system, the lack of darwin support presented a stumbling block. I have implemented darwin-specific getpeereid() and getpeerpid() (see https://gist.github.com/LucaFilipozzi/e4f1e118202aff27af6aadebda1b5d91) as a precursor to adding darwin support to this project. I've created a pull request that could be used to resolve this issue. Thanks!