Set up automatic documentation generation

[djrieger]

Proposal:

1. Someone pushes new commits to dev
2. A hook linking our GitHub repo and travis-ci.com tells Travis CI to start building
3. Travis CI pulls dev, runs doxygen and pushes the resulting documentation to our gh-pages branch

See http://stackoverflow.com/questions/23277391/pushing-to-github-from-travis-ci

[Nidan]

seems impossible according to @djrieger, all tested providers have too old C++ compilers. Alternative solution: Use one of our own machines (via cronjob etc.)

[Nidan]

I've set up a cronjob to regenerate and push the documentation every hour, given changes in dev. Seems to work, but the first real test will be tomorrow.

[djrieger]

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