Update Checkouts guide to include mention of order_token + next action
Replaced italics with backticks/pre, per current style guide.

Fixes #132
Typo fixes and minor copy changes in Integration Overview.

Fixes #133
Updated 'nanoc preview' to 'nanoc view'

Fixes #134
Use number parameter (not id parameter) in shipments API

Fixes #131
[dev] Correct documentation about the total on orders
[dev] Add useful methods to orders guide