Radial linear scale point label font family settings bug

Hi, there is a small bug in master branch in [radial linear scale](https://github.com/chartjs/Chart.js/blob/master/src/scales/scale.radialLinear.js#L357) that prevent setting font properties of point label. I've fixed it in my [fork](https://github.com/msimulcik/Chart.js/blob/master/src/scales/scale.radialLinear.js#L357). May I open pull request?
