Add automatic documentation generation with doxygen via travis

*Do __not__ merge without reading this!*
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First initialize the `gh-pages` by using [this URL](https://github.com/znc/znc/generated_pages/new).

To make this work you have to add an encrypted secret key to `.travis.yml`.
First someone with write access to this repository has to create a new [access token](https://github.com/settings/tokens/new) that has write access to this repository.
Then you have to install the travis gem (`gem install travis`) to be able to store the [encrypted](http://docs.travis-ci.com/user/encryption-keys/) token in `.travis.yml`.
`cd` to the ZNC directory if you're not already in it.
Use e.g. `travis encrypt GH_TOKEN=b0ee9cc6bd7c6035daae75ef9bcfb27a4cdb58a3` to generate the encrypted token.
Now you can merge my PR in your local copy of ZNC change `<<TOKEN>>` in `.travis.yml` to your encrypted token, commit the change and push everything back to master.

This will enable travis to always push the latest docs for the master branch to https://znc.github.io/znc/ as long as all builds pass.