Demo template inheritance

Hi Areski, 

Another PR. This you can definitely cherry-pick ... I'd be happy though if you agree to incorporate all. 
The first commit 9a164ec does some "drastic" changes to NVD3Chart. As we talked about, the previously class properties can be instance properties, so I change that, and added a default key value to the instance property through ```self.attr = kwargs.get('attr', 'default')```. Together with that, I update the documentation. 

Another thing that changed in ```NVD3Chart```, is that some properties that were only relevant to pieChart where deleted. They are added to ```pieChart``` in 92bf9fa. 

f9ff853, is all about cosmetic changes ... namely, pep8.

And finally, 8af0903 is again a proof of concept. I removed from ```content.html``` stuff that was unique to ```pieChart```, it is reappearing in ```pieChart.html``` again. 

Note that jinja syntax offers some really cool tricks. Instead of doing: 
      
      {% if chart.show_labels %}
        chart.showLabels(true);
      {% else %}
          chart.showLabels(false);
      {% endif %}

You can simply do:
 
    chart.showLabels({{chart.show_labels|lower}});

At first look it might look obfuscated, but ... the resulting js has only one line, where the previous code had many empty lines...

We can also do it for ```chart.donut```

          {% if chart.donut %}
          chart.donut(true);
          chart.donutRatio({{ chart.donutRatio }});
          {% else %}
          chart.donut(false);
          {% endif %}

Can be changed to:

          chart.donut({{chart.donut|lower}});
          {% if chart.donut %}
          chart.donutRatio({{ chart.donutRatio }});
          {% endif %}

That's it for now. If you like the concept, I will remove from ```content.html``` everything not common to all chart types, and eventually, I will remove content such that only ```base.html``` will exist . The later step, is just to simplify from 3 templates that each chart is based on to 2 each chart. 
When we have a simple structure of templates, in contrast to one monolithic template full with if else ifs, we can easily integrate more NVD3 Chart type contributed by other users (as for other users contributing, we can only hope...,, but at least we can make it easier for them to contribute...).

Cheers, 
Oz