Add ability to plot on absolute time (Issue #3)

Hi again,

I've tweaked the loading code some more so it keeps hold of the original "days" column. I've changed the UI to choose between timebases. The changes are [deployed here](https://mathew-hall.shinyapps.io/shinyCountryTimeseries/). 

This addresses issue #3 but might break merges; I've renamed the data frame from `df5_melt` to `long`. Let me know if that's a problem and I'll fix in the PR.