added code to shipping methods

better layout

place shipping_method.code on selected shipping rate

Fixes #5439
better way to redefine existing methods: allows to call original behaviour with "super"
Fixes #5358
War on class_eval. refs #5358
Update mail settings configuration change.

Prevents issues getting opened up all over the place about it.
Fixes #5367
Deleted the continue button from adjustment index

On the adjustment index (in the admin) was a continue button.
The button only shows up on this page, it's a leftover from the old 'manual add an order' proces in the admin.

I deleted it.

Fixes #5395
Regression spec for 'stack level too deep' #5384

Fixes #5384
Mention we accept PRs for documentation.
Add in fix from @andrzejpolis Fixes #5384 Fixes #5386
Go to cart and then call next, to trigger hooks
Fixes #5216
Fix specs broken by e6865c6
Fix guides syntax.
Add circle CI setup