Fixed some typos and addressed a few style issues.  The latter point
was mostly about removing the contractions.  This is largely a matter
of taste: How informal are the vignettes meant to be?  Are the
documents too stiff if there are no contractions?  Andy: I will not
object if you overrule some of these changes.

I used TextLint to point out the issues but ignored most of the
complaints. https://github.com/DamienCassou/textlint


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@834 edb9625f-4e0d-4859-8d74-9fd3b1da38cb