Added installation script.

In commit #587c0a637aa1ef99d38e77a68ac870537daa979a, I deleted a
Makefile that acted as an installtion script. At the time, I wasn't
using VM's often and I really never needed to install the dotfiles. Now,
I deal with VM's a lot more and often need to install dotfiles in a sane
way, so I added back an installation script. This time, I ditched make
for plain old shell scripting.