Add charts example using pygal chart library through geetools

[fitoprincipe]

I know there are many options for charts. I choose pygal because I needed to include the chart into a widget. plotly is a great library that allows to include charts into widgets, but what I didn't like was the fact that users have to have an account. After making this module I found bqplot, may be my next effort will be towards that libraty, but by now pygal works fine and I think it is very flexible too!. You can even export the charts into svg or png images.