add new theme based on the factory defaults of the pander package

My [pander package](http://rapporter.github.io/pander/) comes with a feature called "unify plots", which tries to force the same style on any kind of plots generated in R (like base `graphics`, `trellis/lattice` or `ggplot2`), where the default style tries to be very minimal (without box and white-space around the plot etc.). Please see a few examples at the [demo page](http://rapporter.github.io/pander/graphs.html).

This pull request implements a new theme, which can use the default settings of the `pander` package to produce the plots, or can also apply the user's custom settings stored with `panderOptions`, so that it would be easy to enable/disable grid, change the default color palette etc.

I tried to follow the package conventions and also updated the example. Please let me know if there would be anything I should add here. Oh, and I've fixed a small typo of the "Calc Theme" example, I hope you do not mind that :)

This pull request was initiated in the `pander` package repository: [pander/issues/120](https://github.com/Rapporter/pander/issues/120).