Minor update to historytool to handle logs with over 20,000 lines.

Mine just hit 20,044 and the graph stopped updating... There's a new 'last=true' parameter (default is the 'reverse' parameter value, which is passed as false when getting the data for the graph) to make sure it's the last 20,000 records from the log being used.

You can always use 'max_lines' instead but this now allows more careful tuning. (I probably don't need 20,000 records for my 300 day graph, for example.)