skipXLabels - Option to display specified number of labels on X-Axis

Applicable to bar chart and line chart.

User can now pass a { skipXLabels: 10 } to display only 10 labels (actual displayed labels count might be a bit different depending on the number of total labels present on x axis, but it will still remain close to 10 however)

Helps a lot when there is a very large amount of data. Earlier, the graph used to look devastated due to x axis labels drawn over each other in the cramped space. With skipXLabels, user has now control to reduce the number of labels to whatever number of labels fit good into the space available to him.

Please see the attached images for a comparison. the second graph has been passed { skipXLabels: 10 } into option.

![err-graph](https://cloud.githubusercontent.com/assets/2913409/3824433/f1cc7d2a-1d46-11e4-89e9-d55261ff3c46.png)
![great-graph](https://cloud.githubusercontent.com/assets/2913409/3824434/f1cf66ca-1d46-11e4-8a32-e91a641528d3.png)