Deploy gh-pages branch from travis

### WIP for #34 

(this is #44 but merged into a local branch so that we can all push commits to it and we can test the results in this repository)

To safely deploy with Couscous from a Travis build we need to take this things into consideration:

- [x] Make sure the deployment is running only for master.
- [x] Make sure it is not a pull request.
- [x] Make sure we deploy once per build. E.G. don't deploy for every PHP version tested :smile:
- [x] Decide what to use to deploy: [Private Key](http://stackoverflow.com/questions/23277391/pushing-to-github-from-travis-ci) or [Personal Access Token](http://stackoverflow.com/questions/18027115/committing-via-travis-ci-failing)  
    - More info on [Personal Access Token](https://medium.com/@nthgergo/publishing-gh-pages-with-travis-ci-53a8270e87db)
- [x] Write documentation in `docs/travis.md`
- [ ] Couscous specific problem: don't generate & upload `couscous.phar` for pushes to master (else we might push an unstable phar)

The process is detailed in [docs/travis.md](https://github.com/CouscousPHP/Couscous/blob/travis/docs/travis.md)

Feel free to add commits and keep working on this together.
I would inverse the blocks to something simpler (à la *return early*):

```php
if ($input->getOption('php-version') != $currentPhpVersion) {
    $output->writeln('<comment>This version of the ...');
    return 0;
}

// ...
// Generate the website
```

(just like for the other `if`)