Sync changes from spree/spree:frontend up to b07b838.
Move development gem dependencies into the Gemfile.

By following the method used in Spree, we can ensure that the
dependencies and versions used in spree_bootstrap are the same as used
in Spree.
Do not install the bundler tasks in spree_bootstrap/frontend.

Rake fails because the install_tasks method expects to be run in a gem,
but frontend/ is not a gem. Also, the bundler tasks aren't used.
Copy all tests from spree/spree@b07b838:frontend, for full test coverage.
Fix for the calculation of tax on the order details page.

`discounted_cost` isn't valid for tax adjustments, and Spree uses
`amount`.
Fix for generating the test app in spree_auth_devise_bootstrap.

The app isn't currently generated in a working state. This commit fixes
it, enabling the tests to go green.
Sync changes from spree/spree_auth_devise up to 761a9d5.
Document how to run the tests for each of the internal gems.
Split core files into a separate folder to help with merges from Spree.
Sync changes from spree/spree:core up to b07b838.