Coq install script

This includes #212 and replaces #202.

The biggest change of this pull request is that HoTT/coq is now bundled as a git submodule.  The main benefits of this are:
* a self-contained way of tracking which version of Coq we depend on
* slightly simpler instructions for obtaining the git clone of HoTT/coq (in that you don't need to know about branches to get the right version of Coq)

Additionally, since the Travis-CI scripts use the bundled Coq submodule, I've made INSTALL mention those scripts, so that we can just say "run this script to get Coq", rather than a long sequence of commands, if you don't need to configure things by hand.

Questions to resolve before merging this:
* Should we tell users to use the travis scripts (so that we get automatic checking that they don't break), or make separate scripts for them (so that they don't notice travis, and so that it's easier to customize the scripts in the future)?
* Should we mention the script that will obtain dependencies automatically if your system has `apt-get`?
* What should the defaults be?  Install coqide or no?  Separate scripts for building and installing, or all in one script?  Install only coqlight, or all of coq?  Run all the building as sudo, or only the install?  How much should you be able to customize building via the convenience scripts?  Should we build with `-local`, or `-prefix /usr/local/bin` or force the user to interactively choose a location, or something else?