Use RSpec's `allow` syntax instead of `stub`

`allow` was added to RSpec 2 along with `expect` as part of the move
away from `should` and `stub` which monkey-patched `Object`. It will be
the default in RSpec 3.
Suggest a rebase/squash before pull requests

Performing a rebase/squash before a pull request performs an opportunity to
craft well-written commits before reviews are performed. This makes the review
easier, as the commits are easier to understand, and it allows review of the
actual commit titles and messages.
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
Avoid `link_to` for non-GET requests.

* Using `link_to` with the `method` option introduces an un-necessary
  dependency on JavaScript.
* HTML elements -- in this case a button or a link -- should be chosen for
  their semantics, not their appearance.
* CSS can be used to make links look like buttons and buttons look like
  links.
Add js- prefix to classes as well as ids that use javascript
Remove guideline to prepend js- to ids using javascript
Link to Liftoff

Move Liftoff guideline out of style, which is intended to be mostly
formatting, and into best practices.
Coffeescript preferred styles

* Use `@` instead of `this` for scoping consistency
* Prefer double quotes for consistency
Clarify SRP guideline

The single responsibility principle sounds more appropriate when
referring to a class as a whole rather than an object.
Don't use spaces after unary operators

The current guide suggests spaces around all operators, but we typically don't
add spaces after unary operators, such as `!`.

Adopted from #163.