Added showLabelInterval option

Added option to specify a number of XAxis labels to skip. For example, if the chart had 100 data points, you could set this option to 10 and show every 10th data point label on the chart.

This example shows the use of the interval and how it can help to clear up the labels on the X-Axis for rectangular charts.

# Without showLabelInterval
![screen shot 2014-09-11 at 1 07 50 pm](https://cloud.githubusercontent.com/assets/5332920/4239166/aa14177c-39de-11e4-9d93-0c3dd0c2e510.png)

# With showLabelInterval set to 2
![screen shot 2014-09-11 at 1 08 05 pm](https://cloud.githubusercontent.com/assets/5332920/4239165/aa112814-39de-11e4-8dca-bb843f399a0f.png)