Add more bandwidth graphs

So far we have a bandwidth graph based on the sum of advertised bandwidth of all running relays. We should add a graph for advertised bandwidth of all exit relays. Also, we should add graphs on bandwidth histories as contained in extra-info descriptors. For the latter we can add a new *StatsFileHandler class that takes bandwidth history lines from the RelayDescriptorParser class and writes them to a bandwidth-history-stats file.