Respect dates relationships

In https://github.com/gulpjs/better-stats/blob/687b378534678aed0ff7f3a6e91476df93cd6edc/index.js#L42-L58, it show that when a date is not available it uses current time by default, but there are some cases when there're better defaults based on other ones. For example, `atime` can be filled with current time or from `mtime` if that last one is available, and the `birthtime` should be the oldest one if it's not explicitily available. More info at https://nodejs.org/api/fs.html#fs_stat_time_values