Use ruby-mode for Podfile

A Podfile is used for CocoaPods to do a Bundler like dependency
management in XCode projects. Similar to a Rakefile or a Gemfile the
Podfile contains a dependency description in a Ruby based DSL.
Merge pull request #473 from jehrhardt/podfiles

Use ruby-mode for Podfile
Factor the search engine installation to ease the future install of new ones.
Add DuckDuckGo search engine in the default list.
Add binding `C-c U` for DuckDuckGo search
Merge pull request #476 from ardumont/add-duckduckgo-search-engine

Add DuckDuckGo to the search engines list
Merge branch 'master' of github.com:bbatsov/prelude into develop