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
Add header_action method for panels
Use Capybara to better test panel rendering
oops, don't raise an exception!
simplify options handling code for `actions`
style update
make `default_actions` private

note: this is a backwards-incompatible change!
changed spec syntax from should to expect
Merge pull request #3136 from irmiller22/rspec-syntax-change

RSpec Syntax Change
use expect() RSpec syntax in Cucumber steps
simplify ActiveAdmin::BatchActions::BatchActionPopover#action
Merge pull request #3139 from timoschilling/code_simplification

simplify ActiveAdmin::BatchActions::BatchActionPopover#action
support ransack short matchers
remove double if
Merge pull request #3144 from timoschilling/rm_doubled_if

remove double if
Merge pull request #3141 from timoschilling/support_short_predicates

support ransack short matchers