Docstring for NumberColumn.percentile should describe our methodology

[onyxfish]

Because nobody seems to agree on the one true way of calculating this

[jheasly]

Well, the "Estimating the quantiles of a population" section of the wikipedia Quantile entry has a nice roundup of the methods, trouble is, I don't quite know how the method we're using, which comes from
http://stackoverflow.com/questions/2374640/how-do-i-calculate-percentiles-with-python-numpy/2753343#2753343 and
http://code.activestate.com/recipes/511478-finding-the-percentile-of-the-values/ maps to the wikipedia table, otherwise I'd take a whack at it.

[onyxfish]

I think the safest description is "linear interpolation between nearest ranks", which is one method outlined in the wikipedia Percentile entry. It's sort of amazing to me that this isn't what Excel uses. (I checked yesterday and LibreOffice does something else entirely.)