Modifications to pull request #215

This pull request supersedes #215. It does two things:

1. Makes the short installation instructions shorter.
2. Provides a custom `./etc/coq_install.sh` script which does not try to install Coq. This is to be used when following the short installation instructions.

The installation logic is now streamlined, but also automated in the most annoying part (compiling custom Coq):

1. Install prerequisites (OCaml, make, git, etc.)
2. Get the HoTT librarry.
3. Compile custom Coq, either using the provided script or with bare hands.
4. Compile the HoTT library.

I hope we got it right.