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
Amend resource-customization doc
refactor error message for filters
store filters in a hash to simplify implementation

This fixes #2507 and adds tests to ensure that's the case.

When using a hash it's simple to write over an existing filter, and
it's easy to avoid that when adding the default filters in.

Before this commit there was a array of filters that we'd just append,
and later only use the last of for each filter name.
Merge pull request #2515 from Daxter/bugfix/2507-preserve_default_filters!

[2507] Preserve default filters!
update gems where possible (easy)

cucumber & capybara are the hard ones; cucumber-rails is still waiting
on #2388

Gems removed:
sprockets: not needed because Rails itself requires it
guard-coffeescript: not used anywhere
stub! is deprecated; use stub instead
mock & stub are deprecated; use double instead
set up simplecov and make coveralls more accurate

now when you run `rake test` an HTML file is generated by simplecov in
`coverage/index.html` for your perusing pleasure, to see which files
need better coverage
fix wonky problem with minitest & rails 4

see the comments in #2517 for more info