add utcMonthDay, timeMonthDay

[mbostock]

Fixes #62.

I think it’s proven pretty surprising that d3.utcDay.every(3) is not regularly spaced, so better to match d3.utcDay behave the way people expect, and have d3.utcMonthDay for the month-aligned day interval. Same goes for d3.timeDay, and now we don’t need the awkwardly named d3.unixDay (though I’ll keep it as a deprecated alias for backwards compatibility).

[mbostock]

We also need to delete the README here and point to the new documentation on d3js.org, and update the documentation there… I’ll do that separately.

[mbostock]

Broader discussion in #64, wherein I think we should consider changing the behavior of interval.every universally. The main wrinkle is d3.timeHour and daylight savings time… Also, arguably, the design of D3’s “multi-scale” tick format, but I think that becomes less important now that we have a better strategy for time axes in Observable Plot.

[mbostock]

Closing in favor of #66.