Add extra parameters to graphs and missing requirements

Hi, 
Let's start from the easy part: the file ```requirements.txt``` was missing, so I could not run ```make build```.
I added it.

Now for the more complex part: 

python-nvd3 is a great pathway into d3 graphs. Modifying and customizing can be a little bit tricky. So, for those who want a better control on the graphs, I added a keyword ```extras```. I also added documentation for this keyword and example. 

[Here][1] you can find the original motivation, and over this [fiddle][2] you can see a working example:

[1]: http://stackoverflow.com/questions/24533387/d3-text-color-inside-graph
[2]:http://jsfiddle.net/oz123/d2GcC/2/

Yes, you are right! My mistake documenting it. The lines 306 to 310 are actually already generated. So the only line 
that modifies the text color is:

```text_white= d3.selectAll("#pieChart text").style("fill", "white");```

Let me fix it. I will update my pull request later.