Significant new content

I moved some things around. I preserved isomorphismes' "no programming knowledge first" approach; I really like this. In the "programming-heavy" part of the tutorial, I emphasized from the very beginning that every "single" thing in R, like 4 or "foo", is really just a vector of length 1. I added in some whirlwind tours of data.table and ggplot2.
Update r.html.markdown

Minor changes to comments (fixing typos, etc.). Deleted "weird types" section; broke out "NULL" type into its own type category. Added instructions for dropping rows and columns in data.frame and data.table. How to make summary tables in data.table.
Merge pull request #618 from e99n09/patch-1

[R] Significant new content