Use Rserve and servlets to display custom graphs

Part of Kevin's GSoC project was to employ Rserve and servlets to allow users to customize graphs. The current servlet accepts as parameters start and end date, sends a request to produce a graph to Rserve, and returns the graph bytes (or an error code) to the client. Further, there's a dynamic graph page containing an HTML form for users to request a graph.

As discussed on #tor-dev today, we are merging Kevin's code into metrics-web. Further discussion of the merging will take place here.