Basic time-series data support

I added basic support for posting individual values (e.g. the median of a histogram) to plotocrat, and plotting their values over time. Every value can be linked to a URL (e.g. to the histogram of the data behind the value).

Not sure if this functionality makes sense as part of Plotocrat, but I'd appreciate if you could pull in the changes to allow non-`#cdf` plots :)