added plot_timeseries

I wrote a small function to plot relative abundance across a time variable using `geom_smooth`. The example is slow and the documentation needs a little work.

I use a similar function to this all the time and thought it would be a useful addition.

The function I typically use differs in that I plot a `geom_ribbon` with the standard error around the mean. I also discretize the time variable using `cut`. I would add this but I thought it would be better to stick to plain ggplot2 style plotting.

Let me know what you think. I can add more docs and create a better example.