Several improvements and refinements:

- Reintroduced full support for missing values (sufficient statistics on available cases)
- Added layering to mmhc
- Added mmhc as an option in boot.bn (and restyling of the whole function)
- Added package Rgraphviz as 'Suggested', with the specific requirement in the plot.mat function
- Commented some print, printf and cat around the code
- Removed the file exdata.R