Remove "test user needs createdb privilege" workaround.

It doesn't seem to work any more in Rails 3.2. Just accept that your
test user needs to own the test db and have the createdb privilege.

ALTER ROLE arvados_test_user CREATEDB
Add "not null" constraint to timestamp columns.

"When the timestamps method creates the created_at and updated_at
columns, it makes them non-nullable by default."

http://guides.rubyonrails.org/3_2_release_notes.html
Preload all models at startup. Otherwise, in non-production mode,
Rails crashes trying to instantiate cached objects before loading the
relevant classes.

http://aaronvb.com/articles/37-rails-caching-and-undefined-class-module
Use current_user to select API response template. fixes #1594