Helper functions for vignettes:
* latexDate(): Use this to fix date shown in document at Sweave / knit time
* latexify(): When you are not directly in control of 'expr' in
  '\Sexpr{expr}', use '\Sexpr{latexify(expr)}', because as.character(expr)
  may include characters that cannot be used in LaTeX code unescaped.


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@862 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
"Interestingly", \Sexpr behaves differently in knitr vs Sweave.
Added an option, 'doublebackslash', to latexify().


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@863 edb9625f-4e0d-4859-8d74-9fd3b1da38cb