Web stats

Adding some basic stats to web.py, as agreed on the mailing list.

I am using Google Charts as they are completely free (unlike HighCharts), the main disadvantage is the fact that they require you to load the library directly from Google, you cannot load them from your local storage. However, changing the library is as simple as defining a new "stats.html" template; the data extraction backend can remain the same.

For the moment I am only adding stats and graphs about tasks and machines, I am leaving out all the part regarding RRDtool and disk loads for now, as it would involve installing external libraries.