handle asset gems in a sane manner

Instead of having hard requirements for asset-related gems in our
gemspec, this approach allows us to require them as needed.

+ use Bundler to confirm that asset gems are installed
+ remove asset compilation code that's no longer needed
remove support for Rails 3.0
update asset generator and tests for a post-3.0 world

+ changed the JS template file to use CoffeeScript
+ added a few tests to ensure core AA files are created by the generator
Merge remote-tracking branch 'daxter/bugfix/sane-asset-gem-handler'

Conflicts:
	.travis.yml
	Gemfile
	activeadmin.gemspec
	lib/active_admin.rb
	lib/active_admin/dependency_checker.rb
	spec/unit/rails_spec.rb