Adding Continuous Integration

[pimotte]

This adds a bare-bones, functional .travis.yml to add continuous integration.

For this to function on the discipl/core repo, permissions need to be given through: https://travis-ci.org/organizations/discipl/repositories

The permissions asked for will be:

• Read access to code
• Read access to metadata and pull requests
• Read and write access to administration, checks, commit statuses, and deployments

The last one enables GitHub to show the status of Travis builds, which need to be written to GitHub by Travis.

Sample build with this branch: https://travis-ci.org/pimotte/core/builds/466513436

[bkaptijn]

Enabled travis to do his job. Added build status badge in the readme.