JFG-767 Create package for CSV export. Implement exporting of plots

+ create download servlet
+ plot representation know what metric node it represented

https://docs.google.com/a/griddynamics.com/drawings/d/1pJEWCmi684jQbpQHq8IDQ2ZjPcQoK2lZtpt9yab6Pi4/edit
Thank you. There is no need in this method anymore, as PlotRepresentaions knows what MetricNode they represents.