add library(knitr) to make sure the functions inside knitr will always be available

sometimes the user might not have loaded the knitr package: http://stackoverflow.com/q/17734135/559676
knitr-slides have been removed
use my style: single quote, wrap lines, 2 spaces indent, spaces around operators
rocco.css should not be placed under themes/, because themes/ is for syntax highlighting
example not satisfactory; will do it later
provide an HTML template for markdown so that we can use the template and stylesheet arguments

title and style are no longer necessary because they are present in markdownToHTML()
the background div does not work well when there is graphics output
make sure images in docs are not too wide
no margin for container; width of docs/code: half and half (important when there are plots)
add Weicheng Zhu @mingsnu to contributor list for #577
add a reference to Docco in rocco()
news about #577
roxygenize
bump version