Setup end-to-end tests.

[ntarocco]

• re-structured tests packages to separate unit and e2e
• added selenium and pytest-flask dependencies
• setup conftest.py for selenium tests
• take screenshots when selenium test fails for easier debug
• added a simple test case to check the homepage
• added a simple test case for login form
• updated .travis.yml

[lnielsen]

Awesome, looks great with the e2e tests :-) Just minor comments above.