add scale_fill functions

Hi Jeffrey -
only small changes, please feel free to refuse

See email from 18 September:
Hi Jeffrey,

thanks for this package - good initiative!

I was testing the examples at https://github.com/jrnold/ggthemes with a bar chart and realized the scale_fill_fivethirtyeight() was missing - it's a minor modification but I don't know github well enough to implement the change myself (except forking etc)

let me know your suggestion,

thanks,
Bo