Equivalent of d3.unixDay for local time

[mbostock]

Ref. #58 (comment)

Also, it’s a bit confusing that d3.unixDay doesn’t have the d3.utc prefix like the other UTC time intervals. I wonder if it wouldn’t make more sense to

1. Rename d3.utcDay to d3.utcMonthDay
2. Rename d3.timeDay to d3.timeMonthDay
3. Rename d3.unixDay to d3.utcDay
4. Create a d3.timeDay that is analogous to the new d3.utcDay (formerly d3.unixDay)

This would make sure that d3.timeTicks is consistent with d3.utcTicks, and eliminate the occasional irregularly-spaced tick at the start of the month.

[mbostock]

Folding into #64.