Autobuild documentation with travis

Now that we have a `make html` target, we can make the html
automatically on travis.  If travis builds something which is the same
as origin's master, then we autocommit updated documentation and push to
the gh-pages branch, where documentation is accessible at
http://hott.github.io/HoTT/coqdoc-html/.

Generating the files was a pain.  I record the process here, so that if
we need to update the keys, there will be a reference.

First, generate the keys that will be used
```bash
$ ssh-keygen -t rsa -C "Travis-CI-bot@travis.fake" -f "$(readlink -f ./id_rsa)"
```
When it prompts you for a passphrase, you must leave it blank.

Then add the contents of ./id_rsa.pub as a deploy key to the HoTT
repository at https://github.com/HoTT/HoTT/settings/keys.

Now, following a modified version of
http://about.travis-ci.org/docs/user/travis-pro/#How-can-I-encrypt-files-that-include-sensitive-data%3F,
encrypt the id_rsa file:
```bash
$ password=`cat /dev/urandom | head -c 10000 | openssl sha1`
$ openssl aes-256-cbc -k "$password" -in id_rsa -out id_rsa.enc -a
```
Now, the hard part (at least if you don't have sudo). Hope that you
either have ruby and gem installed, or a version of ssh-keygen which
supports the `-e -m` flag combination (check with `ssh-keygen -\?`).  If
you have ruby and gem, then you can follow
http://about.travis-ci.org/docs/user/encryption-keys/, and run
```bash
$ gem install travis
$ travis encrypt -r HoTT/HoTT "SSHKEY_PASSWORD=$password"
```
and update the `secure:` part of the `.travis.yml` file with the resulting
secret.  Otherwise, you can follow
http://about.travis-ci.org/docs/user/travis-pro/#How-can-I-encrypt-files-that-include-sensitive-data%3F
and run
```bash
$ \curl https://api.travis-ci.org/repos/HoTT/HoTT/key | perl -pe 's/^{"key"://; s/^"//; s/"}$//' | sed s'/\\n/\n/g' > id_travis.pub
$ ssh-keygen -e -m PKCS8 -f id_travis.pub > id_travis.pub.pem
$ echo "SSHKEY_PASSWORD='$password'" | openssl rsautl -encrypt -pubin -inkey id_travis.pub.pem
```
However, if `ssh-keygen` fails, complaining about `-m` being
unrecognized, then you must clone or copy
https://gist.github.com/thwarted/1024558
(https://gist.github.com/1024558.git), install the required python
dependencies (for me, pyasn1), and then run that on `id_travis.pub` to
get the `id_travis.pub.pem` file, and then run the `openssl rsautl`
commmand.  Either way, the output of that third command should be placed
in the `.travis.yml` file, replacing the existing secret.

Perhaps we should include these instructions somewhere in the
repository?