Mosaicplots in the ggplot2 framework: ggmosaic

The package productplots already provides a fully functional base for creating mosaic plots based on the ggplot2 framework. I will make use of this in two ways: (1) rather than building mosaicplots from scratch, I will use the data handling provided in the productplots package and only implement the additional ggproto interface to make mosaic plots a full geom that can be used as one of the layers in a regular ggplot2 chart. (2) the existing functionality of the productplots package will give me a test basis for the first part of the project to test output from the ggmosaic package. In a first step I want to implement all of the productplots functionality in form of a geom. In a second step, I will address the way groups are handled (which are used for facetting and/or filling) to allow for functionality orthogonal to variables used in the formula call, which at the moment is not possible in productplots.