Add a KeyOnlyTextOutputReader to enable streaming to write out text files without separators

MR jobs sometimes want to just output lines of text, not key/value pairs. TextOutputFormat handles this by, if a null value is given, outputting only the key with no separator. Streaming jobs are unable to take advantage of this, because they can't output null values. A text output format reader takes each line as a key and outputs NullWritables for values would allow streaming jobs to output lines of text.