Update path to github so that it works

This unbreaks the copying of files from github in the absence of
autoreconf.
Don't add files to git index; make travis check

Now, travis will (hopefully) actually try to build this without
autoreconf.  Also, ./autogen.sh no longer adds files to the index.
Only fallback on git if autoreconf not found
Bundle coq as a git submodule

This will hopefully pave the way to make it easier to build coq for new
users.
Update travis to build the git submodule coq
Update INSTALL.txt

It now explains how to use the travis scripts to install Coq
automatically, and how to use the bundled version to not have to manage
branches.
Make separate etc/install_coq.sh script for users
Improve installation instructions
Remove useless -j
Incorporate @JasonGross's suggestions
Merge pull request #216 from andrejbauer/master

Modifications to pull request #215