Update of NVD3Chart

Extend documentation, and make most class attribute instance
attributes.
Add attributes related to pieChart

* These were removed earlier from NVD3Chart base class
pep8ify
Introduce template inheritance
Merge pull request #70 from oz123/demo_template_inheritance

Demo template inheritance
Merge branch 'develop' of https://github.com/areski/python-nvd3 into develop