build, starter & all: add support for e2e testing

[dsebastien]

Goal: re-integrate our e2e testing support.
Should be done at all levels:

• project root
• each package
• starter
• demo

[RobbyDeLaet]

e2e testing is in place in the starter template app and in the showcase app.
In the packages we don't need to support e2e tests. All components are tested in the showcase app.

[dsebastien]

@RobbyDeLaet and what about the protractor config? Do we have a default one in stark-testing or no need for it?

Also for e2e, I'd like to have coverage reporting and I think that'll require a bit of config, also for coveralls?