Adds support for append_point ECA function.

The add_point ECA function pushes a data point to a chart, but then
the HighCharts API call in js/handlers.js applies the 'shift' flag.

Since both options make sense, the newly added append_point ECA
function satisfies the 'non shifting' data addition, this allows the
correct operation of bar charts, and of growing line charts.
Merge branch 'master' of https://github.com/utwente-db/eca-rule-engine