Added prelim presentation.

  Modified the makefile, and removed all the R files. Now, rather
than regenerating all the plots, they just are kept in the repo.
  Added a prelim/.gitignore, to ignore the presentation pdf
generated when make is called.
  Removed the top-level .gitignore, because all it was doing
was ignoring the 'prelim' directory, which I now want to be
part of the repo.