* tests: use stages for the travis execution

Fixes #694

* add comments as suggested by @nventuro

* move the unit tests first, as suggested by @frangio

* make stages clearer as suggested by @nventuro

* tests: use stages for the travis execution

Fixes #694

* add comments as suggested by @nventuro

* move the unit tests first, as suggested by @frangio

* make stages clearer as suggested by @nventuro

* update the stage names as suggested by @frangio

* Run all tests in parallel

* added names