use keepalive in Legend.AddEntry
Merge pull request #560 from ndawe/master

[MRG] use keepalive in Legend.AddEntry
ratioplot template
add fill_value argument to Hist.divide
RatioPlot: set tick lengths
RatioPlot: draw horizontal lines in ratio pad
_limits_helper: don't raise ValueError and instead return reasonable values if min=max
templates: center ratio plot y-axis title