/*
 * call-seq:
 *    readline(sep = "\n")
 *
 * Reads one line from the stream and returns it (including the separator)
 *
 *    reader = Bzip2::Reader.new Bzip2.compress("a\nb")
 *    reader.readline # => "a\n"
 *    reader.readline # => "b"
 *    reader.readline # => raises Bzip2::EOZError
 *
 *
 * @param [String] sep the newline separator character
 * @return [String] the read line
 * @see Bzip2::Reader.readlines
 * @raise [Bzip2::EOZError] if the stream has reached its end
 */
static VALUE bz_reader_readline(int argc, VALUE *argv, VALUE obj) {