Reduce repo size

[valleyjo]

Not sure if this is worth doing. With the addition of the new lecture files, the old ones are still stored in the .git/objects directory. The repo is pretty big (~320 MB) and looks like it could be reduced to ~80 MB by removing the old pdf-lectures from the pack.

Here is a tutorial. For filename, just say *.pdf. I don't think I can put in a PR for this because it requires a force push at the end.

Again, not sure if this is worth doing but the repo does take a bit to clone.

[D-Land]

This is also a really nice tool (written in scala) http://rtyley.github.io/bfg-repo-cleaner/ for removing large repo files that are no longer contained in the head. It's recommended by github cause it's faster than git filtering and such.

[laboon]

I'll take a look at this on Friday, thanks!