Implements streaming CSVs.

Allows unlimited downloads of CSV data using ActiveRecord batch APIs and ActionController response streaming.

Works on Rails 3.2+
Actually pass the options to generate_line.
Make the meat and potatoes of streaming protected.
Add no-cache header to streaming CSV responses.
Disable downstream response buffering of stream
Cache Bundler to S3 on Travis
Workaround for CSV#generate encoding issue.
Add integration tests
maybe the latest Bundler isn't needed anymore?
Cache the test app to S3 on Travis
turns out tar remembers the folder structure!
create proper URLs
actually save files when `:as` isn't provided
update `rake setup` to not run if test app exists
Add encode options
Add header_action method for panels
Use Capybara to better test panel rendering
add no-default-user option to active_admin:devise generator

When using active admin with a customized User model living in the
main app, it is common to add further validations for profile
attributes etc. Creating a user in the generated migration may fail
then since not all required attributes are present.

In those cases it's better to setup the initial user via a database
seed. This option allows other engines, which bundle active admin, to
invoke default active admin generators while still providing their
default user via a seed.
resolve #3118
Docs: custom authentication now more obvious

I was having a bit of difficulty figuring out where exactly to implement my own authentication method, until I found http://dan.doezema.com/2012/02/how-to-implement-a-single-user-model-with-rails-activeadmin-and-devise/ and realized I should be implementing them in ApplicationController.

Hoping this helps fellow ActiveAdmin newbies.