Include addons code from travis-core, remove travis-core dependency

This builds on the ideas in #6, but it takes a simpler approach.

It focuses on moving the required parts of travis-core over to travis-tasks, without changing much just yet. Compatibility is the key I wanted to get out first before we focus on refactoring what travis-tasks is or should be.

I was overwhelmed with everything present in #6 and wanted to start off simpler so we can slowly build on this, start moving things around, renaming it slowly but steadily. This should remove travis-core from travis-tasks quicker so we can start reworking things based on this.