begin and end parameter for chart servlet added

New pull request, because I have uploaded a wrong file in my first pull request.

Add support for the chart parameter "begin" and "end" in this format yyyyMMddHHmm (y := year, M := month, d := day, H := hour (0-23), m := minute).
To avoid ambiguous you are not allowed to specify the three parameter period, begin and end at the same time.