Add support for rnoweb filetype

Rnoweb is a filetype that combines LaTeX and R. It's used by systems like Sweave and Knitr.
Add in r.snippets file

I converted these over from the snipMate r.snippets in the snippets/ directory and removed some ones.
Add R snippets to Rnoweb
Tidy up some parts of r.snippets
Add a few more snippets, fix errors as per #332

Hopefully this looks better, @SirVer!
Remove duplicate snippet
Remove trigger from the end of the description
Merge pull request #332 from jdavis/r

New R and Rnoweb snippets