Ci setup

[stijnvanhoey]

This PR adds automatic builds to the lifewatch blog repo and updates the gh-pages branch when commits are pushed to the master branch.

The building is as follows:

• on all branches and during PRs, the website is build with the most recent commit
• when pushed to master, the website is build and deployed to the gh-pages branch

I did not limit the building to open PRs only but for each commit, as the build result will be available in the background when needed or during PR, but won't bother during development.

[stijnvanhoey]

@peterdesmet remark that automatic builds when the underlying theme is updated is - probably - only possible the other way around:

• adding CI to the eurasian-spoonbill theme repo
• provide a trigger in that travis file that initiates the downstream (i.e. this repo) build