Custom chart attribute

This is to allow users to pass a custom chart attribute, which is not yet hardcoded in python-nvd3.
Example:
char_attr = {'xScale':'(d3.scale.log())', 'xAxis.tickValues':'[0.001, 0.01, 0.1, 1, 10, 100]'}
will allow displaying a lineChart with log X axis and tick values shown in the list.

I also implemented this in django-nvd3.