Fauxton: fix missing leading zeros in logs

This adds leading zeros to the times in the log view. Before that
the times were displayed like 1:5:3 (if it were 01:05:03)
because getHours() and friends are returning a Number and not
a String with a leading 0. In the case of 1:5:3 the corrected
time is now displayed as: 01:05:03. As d3 has a nice date formatter
we do not have to roll our own.