scales should be able to wrap

[qgeissmann]

It would be useful to have the labelers wrap time in the x scales, optionally.
e.g.
scale_x_hours gives breaks at:
[0, 6, 12, 18, 24, 30, 36]
we would get instead:
[0, 6, 12, 18, 0, 6, 12]

[qgeissmann]

metadata <- data.frame(id = sprintf("toy_experiment | %02d", 1:20),
                   condition=c("A","B"))
dt <- toy_activity_data(metadata,3)
pl <-  ggetho(dt[t < days(2)], aes(y=asleep)) + stat_pop_etho()
pl + scale_x_hours(time_wrap = hours(24))