fine-tune width of spectrum bands.

Using this as a guide: http://en.wikipedia.org/wiki/List_of_WLAN_channels#2.4.C2.A0GHz_.28802.11b.2Fg.2Fn.29

Channels 1-13 are each 5MHz wide.

Channel 14, and the band below channel 1, are each 12-5/2 MHz wide.
now with time axis.

The time axis is displayed only if the loaded spectrum file has tai64n timestamps, of the sort prepended by multilog(1). More on TAI64: http://cr.yp.to/libtai/tai64.html

However, tai64n parsing is currently broken. We lose some precision and the timestamps are off by a bit as a result.

Also, the time axis looks like crap.
fix: remove extra margin below time axis
fix: spectrum band highlights need offset for time axis.
making time axis a bit nicer.
fix tai64 time parsing: needed leap second offset.
new screenshot.