Add font and color customization for xLabel and yLabel in PNLineChart
Dissociate the xLabelWidth from the lines drawing

* For graphic with a lot of data, we can't display all xLabels. Now it's possible to display a fewLabels with a lot of value.
* Add method to specify the width xLabel.
Update podspec to v0.6.0
Fix bug
Merge pull request #95 from Antoine4011/custom-style

Label customization in PNLineChart