fix(chart.tooltip function): replaced : chart.tooltip.Content with

[aniketmaithani]

• chart.tooltip.contentGenerator
• ref : `http://stackoverflow.com/questions/35125110/django-nvd3-chart-tooltipcontent-is-not-a-function

[coveralls]

Coverage remained the same at 81.034% when pulling de6abf5 on aniketmaithani:develop into 9408cee on areski:develop.

[areski]

related to #128, #118 and #124
There is some work in progress as you can see here:
https://github.com/areski/python-nvd3/pull/128/files
So you might want to create your pull request against the branch nvd3-1.8

[alaalqadi]

@areski please merge this as i need to reflect the chart.tooltipContentGenerator changes ASAP please

[daadu]

@aniketmaithani changing chart.tooltipContentGenerator -> tooltip.contentGenerator is half of the problem solved, the arguments in the callback function have changed from function(key, y, e, graph) to function(d, elem) (Check: https://github.com/novus/nvd3/blob/447cce8c180a92fcabd571cca410760deb3a2c1d/src/tooltip.js#L60) therefore the tooltips are not visible and showing Uncaught TypeError: Cannot read property 'point' of undefined error.

[areski]

Fixed by #166