Reformat links in lists to use reference style

* Prevents wrapping long list items, improving readability
* Removes the need for short URLs
Add rule avoid `any_instance` in RSpec.

* `any_instance` promotes depending on concretions.
* `any_instance` hides complexity.
* Pure stubs promotes isolation and dependency injection.
* Using dependency injection promotes inversion of control.
Add more indepth style for Sass
Move Trello guide into Playbook

A more recent version of this guide has been moved and edited in the
playbook:

https://github.com/thoughtbot/playbook/blob/master/book/one-weeks-iteration.md

Rather than maintain two versions and potentially cause confusion about
where to look for the process, I think we should stick to just the
playbook version.
make it more clear what do do instead of protected
correct language
Refine language.

When I read the original version I thought it was ambiguous: Should I avoid
defining protected methods, or calling them?