refactor AttributesTable#content_for
refactor AttributesTable#content_for specs
add spec for settings setter
add support for `config.setting_name?`

This adds a ActiveModel like questionmark attr checker:

  inheritable_setting :foo, true
  config.foo? # => true

  inheritable_setting :foo, false
  config.foo? # => false

  inheritable_setting :foo, ""
  config.foo? # => false

  inheritable_setting :foo, "bar"
  config.foo? # => true
Merge pull request #3540 from activeadmin/setting_with_questionmark

Setting with questionmark
Merge pull request #3528 from activeadmin/refector_attributes_table

Refector attributes table and his specs
add byte order mark option to csv
Merge pull request #3519 from activeadmin/csv_byte_order_mark

BOM option for CSVBuilder