Associations guide: Add note that you must use the build_ prefix to build associations of the belongs_to and has_one type

Add missing colons on the end of before_initialize and after_initialize documentation in configuring guide

Use ARel for Range Conditions section, remove Date & Time section because users should *never* do that.

Add "Configuring Middleware" section to config guide, starting with mentioning what every bit of middleware does.

There is no RAILS_GEM_VERSION environment variable or constant for Rails 3, since this is now managed by Bundler