HTTP over UDP friendly

I'm working on a library in Rust for use with the UPnP discovery protocol SSDP. This uses HTTP-ish over UDP.

I'm proposing a couple of changes:

1) Don't require a specific TcpStream in the BufferedStream so it's possible to use your MemReaderFakeStream.  
2) Export the MemReaderFakeStream as it's useful in usecases such as mine -- I do agree this would be good to see pushed upstream if you'd rather go that way.

Happy to make changes to get this accepted.

FYI - these changes are working here: https://github.com/davbo/rust-ssdp/blob/master/src/lib.rs#L36