WIP beginning_of_seconds and end_of_seconds

[nathanl]

These are two functions I found useful in my work, where I needed to "round" some datetimes to the nearest hour. They seems to fit in with functions like end_of_month, etc.

This is WIP, as it needs to be split up into protocol implementations. But I wanted to see if you like the idea before doing that.

Summary of changes

I'll review the commits, so I mostly want to understand the "why" rather than the "what"

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit

[nathanl]

This would be split into the various protocol implementations

[coveralls]

Coverage decreased (-0.2%) to 69.816% when pulling 54400d5 on nathanl:nathanl/beginning-and-end-of-seconds into 4711f9b on bitwalker:master.

[bitwalker]

My feeling is that we'd be better off adding Timex.round/2 which rounds to the nearest granularity (as seen in other APIs, like compare/3). A Timex.round/3 variant could take an argument which indicates the rounding direction. Alternatively, we could do Timex.ceil/2 and Timex.floor/2 which act like their mathematical counterparts, but take a granularity like the proposed round/2 function. Thoughts?