Factor out the common parts of of the hoq* scripts

Also add hoqdep
Move hoq-config script to top-level

This way, we can copy it more easily.
Bundle coq

This will hopefully pave the way to make it easier to build coq for new
users.
Auto-init the bundled coq on autogen.sh
Call HoTT/coq configure on ./configure
WIP bundle coq
Fix symlinks
Add some more symlinks to fix anomalies
Update configure to make coq correctly
Only build the bundled coq with travis
Update INSTALL a bit

Now it talks about the bundled Coq.  The extended instructions might be
a bit out of order.
Fix the install target
Fix .gitmodules
Fix INSTALL.txt numbering
WIP Use coq
Generate the symlinks in configure again

I don't see any other way to permit the use of either the bundled coq,
or a system one; we might not have the version of OCaml used to compile
the system Coq.
Test with bundled and global Coq on travis
Improve symlink code