get search method validation "working" with Ransack

With MetaSearch, if you asked if the search object responded to a
method, that meant that it was a valid search method. Ransack doesn't
behave that way, so all we can really do is guess.

Ransack instead returns true if the method is valid **OR** if the
method equals the name of one of the DB attributes, which in my opinion
is completely useless.
update code comments for accuracy & succinctness
Merge pull request #2496 from tomprats/batch-actions-docs

Update batch actions docs
shorten up the first section in CONTRIBUTING.md
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