Prevent addition of duplicate resources in Rails 4

Rails 4's ActiveModel::Name class no longer inherits from String and
breaks the expected comparison behavior when searching for existing
resources.

Redefined `ActiveAdmin::Resource::Name`'s `#eql?` and `#hash` methods to
ensure Name objects can be used as Hash keys.
Remove IH overrrides

Now the shorthand syntax works:
```ruby
controller do
  # Redirects to index page instead of rendering udpated resource
  def update
    update!{ collection_path }
  end
end
```
Merge pull request #2001 from Daxter/bugfix/1930-inherited-resources

[1930] Allow IR shorthand for redirection
update README, since the default install comes with coffee
update the changelog
update sidebar documentation
change wording in changelog
remove old dashboard code that's no longer used
Amend 'Create a new Page' section of 'Custom Pages' doc
Make rows about 80 chars long
Merge pull request #2511 from AlexVPopov/documentation

Amend 'Create a new Page' section of 'Custom Pages' doc