Improvements to ecdfplot

[sethaxen]

This PR makes several improvements to ecdfplot:

• allow setting seriestype to something besides steppost.
• if seriestype is not steppost, don't plot the extra point on the x-axis.
• don't plot non-unique points. e.g., when plotting the ECDF of a Binomial(50, 0.1) sample, at most 51 points should be plotted, but previously it plotted length(x) points.
• allow specifying maximum number of points to plot, which is important when the sample is very large.
• add a docstring

[sethaxen]

I'd also like to add the ability to provide the evaluation points as x, so one could do ecdfplot(0:0.1:1, rand(100)).

[sethaxen]

Before this is merged, I'd like to add the ability to specify the evaluation points.