Remove guide for using single mailer

This is not a guideline we often follow. In addition, it's fairly
restrictive and leads to poor organization of information that could be
better expressed with proper naming. If anything this guide violates our
other guide for naming things to reveal intent.
Add a period
Order ActiveRecord associations above validations

* Mirror class order in tests
Add space around operands in Sass
Fix
Prefer double quotes in Ruby

This will reduce the number of changes we have to make: when a string
transitions into something that needs interpolation, we do not need to
change the quotes.

This will also bring consistency to our writing: all strings will now
have double quotes, regardless of their contents.

It also puts us more in line with a large amount of external Ruby code,
which tends to use double quotes.
Fix Git protocol link

Rebase workflow changed location to inside of protocol/git/README.md
Update Objective-C style guide
Prefer `!` over `not`
Update README.md

Corrects link to the indentation example on the Haskell section.
Add guideline on symbol to_proc

* Using &:method_name is brief but clear
* Using it consistently aids readability
* Preferring simple method calls encourages simple methods