Rails 4 upgrade

Following the official (but still in progress) upgrading guide:
http://guides.rubyonrails.org/upgrading_ruby_on_rails.html

Mostly changing methods arguments to adjust to new signatures, adapting
to new conventions (new-style hashes) and replacing deprecated methods
and configurations.

Gemfile updated accordingly.
Micro: enabling Turn / pretty tests

For some reason the pretty printing for tests went kaput when doing the
upgrade to Rails 4. Since Turn was already on the Gemfile from the
start, enable it to run tests with other pretty printing, but
different, still pretty.
Merge pull request #2 from redealumni/upgrade_rails_4

Upgrade to Rails 4