Days diffs are calculated based on the beginning of the day

[fracmak]

Fixes #52

Because day diffs are calculated based on the time between the from and to day, rounded up or down, you could end up saying one day is "yesterday" when it really occurred 12 hours ago on the same day. To fix this we normalize the days to midnight in the current locale and then convert it into milliseconds to calculate the difference in days.

Test included to show that diff between midnight and 1 second before midnight should result in 'yesterday'

[yahoocla]

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[sdwvit]

@okuryu pls review & merge, this is critical for business

[caridy]

It is a little more complicated that just this, read https://github.com/tc39/proposal-intl-relative-time/issues/9 for more details.