.Rbuildignore should be part of the repository
ignore .gitkeep files for building, but assert that empty directories are included when building
hook_plot_html has been exported from knitr almost from day one
avoid bogus warnings by R CMD check

- create helper function slidify_or_markdownToHTML
- leave out unnecessary `require(markdown)`
remove unnecessary knit:: prefix
use anonymous function for .onLoad again
Merge pull request #317 from krlmlr/warnings

Remove warnings from R CMD check