Adding some Highcharts JS themes

Hi jrnold,

Time ago I writed a post about a [Highcharts theme in ggplot2](http://jkunst.com/blog/post/ggplot-highcharts-taste/) and I have a few feedback comments and one of them was about to make a pull request to your package, so here I am.

About the commits I have made:

1. Add R/hc.R file which contains the theme. The template was wsj.R
2. Modify NAMESPACE and DESCRIPTON files. R/ggthemes-data.R to add the backgroud colors and palletes. There are for now 2 themes: defaul, and darkunica.
3. I tried modify vignettes/children/examples.RMD but I can't compile the readme.md (makefile) :(!

Well, that's it. Tell me what you think.

Thanks in advance. Have a nice day.