allow negative dates, y labels, better spacing

[rneher]

This fixes the long-standing issue #483 and other non-documented problems with the tree grid. It now allows

• negative dates (yes, there were pathogens before Christ)
• adds ylabels indicating divergence in the clock display. This could use some tweaking
• improves the spacing of grid lines
• removes the decimal points from dates like 2015.0 (unless major grid resolution is <1y).

[jameshadfield]

Looks great -- thanks @rneher

I added some comments and cleaned up the code a bit in c7393ea

And then I added horizontal grid lines for clock-layouts in 289add4. If this isn't a good direction to go, i'll remove the commit before merging into master.

[rneher]

great. I think the horizontal grid lines are useful.

[trvrb]

I really like the horizontal grid lines for clock display. This labeling makes clock display much less opaque.

[trvrb]

Actually... I'm getting a bug where if I toggle back and forth between "rectangular" and "clock" the horizontal grid lines will appear in front of the tree.

[jameshadfield]

Ahhh merged without seeing that message! I'll fix the bug now. Update: see #612