Update R/utils.R

Updated enable_cdn to use slidifylibraries.googlecode.com rather than slidify.googlecode.com, fixing bug with publishing slides 
Merge pull request #165 from richarddmorey/master

fixed bug with publishing - cdn url was wrong
add logo to README
.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)`
Merge branch 'master' into master-dev2

Conflicts:
	R/utils.R

- resolved by changing slidify.googlecode to slidifylibraries2.googlecode in URL
Merge branch 'warnings' into warnings-dev2

Conflicts:
	R/process.R
	R/zzz.R

- Change to `process.R` does not apply anymore
- Conflict in `zzz.R` resolved by leaving out parameter `outputFile` that has been removed
    - Note that this assumes that RStudio follows the same logic as slidify when determining output file name from input file name; this may or may not be true or even change in the future, why has this parameter been removed from `slidify()` in the first place?
remove unnecessary knit:: prefix
use anonymous function for .onLoad again
Merge branch 'warnings' into warnings-dev2

Conflicts:
	R/process.R
	R/zzz.R

- First conflict doesn't apply, because affected code is not in the dev branch anymore
- Resolved second conflict by again removing parameter `outputFile` from call
Merge pull request #318 from krlmlr/warnings-dev2

Remove (some) warnings from R CMD check