asset guide, match application.rb snippets to the generated file [ci skip]

`if defined?(Bundler)` is no longer used in our current application.rb
fix HashWithIndifferentAccess#to_hash behaviour
Rails 4 has deprecated :order in has_many to use a lambda instead, update the guide accordingly
Fix typo "can exploited" with "can be exploited"
Add a few overview bullet points
Clean up the wording in a few places
document String#to_time exception
fix typo
Simple grammar updates
added to rdoc for unscope that default_scope wins
Capitalizing internet to Internet. [ci skip]
replace apostrophe with backquote for matching [ci skip]
Add some documentation for AM.mailer_name
Fix incorrect post_path link generation.
Remove unopened closing brace.
Fix another incorrect post_path link generation.
Update CreateComments migration to reflect the output from Rails 4.
comments in the callbacks description are in present tense
Changing the wordings in a few places
dir -> directory