demux: Fixed the Keys fn to not return a reference (#3)

Merge pull request #2 from cycloidio/fg-delete

demux: Changed the NewDemux to expect interface

This way the caller can use any of the io.ReadWriter implementations of the Mux and at the end
we have to cast it back to *mux so we do not care of that specific type

README: Improve it also with the rename missing

Rename from 'writer' to 'mxwriter'

So it's easy distinguished from a normal 'writer'