Add from __future__ statements in all files

[giovannipizzi]

This fixes #2040
Note that we did NOT add unicode_literal as it was doing more harm than good (see discussion in the issue). Instead we added a few more, different ones (absolute_import, division, print_function).

[coveralls]

Coverage increased (+8.3%) to 68.34% when pulling 02c3168 on giovannipizzi:fix_2040_unicode_literals into a3e8bbf on aiidateam:develop.

[coveralls]

Coverage increased (+0.5%) to 68.34% when pulling 8a1aa99 on giovannipizzi:fix_2040_unicode_literals into c149362 on aiidateam:develop.

[sphuber]

If you feel like it, one could add a small check to remove empty lines above the imports, but that is of course just for the bonus points ;)

[giovannipizzi]

That's easy, but I will do another time ;-)
Let me know if I should update the branch and merge or you will do it, thanks!