Add enviromental variable to allow smoke testing

running in node with the environmental variable READABLE_STRAM set to 'disable' will cause this module to simply return the platform streams. The goal of this is to allow us to test changes to node streams and see if they will break modules using readable-stream. Transforms have been added so that in browserify (where the streams module is build on readable-streams) this change will be removed and not bloat the build.