title: Add run script build phase to Xcode project from podspec
body: <p>I'm trying to write Cocoapods specification for my library which must modify Xcode project and add "Run Script Build Phase" to project's target. I thought I can use <a href="http://docs.cocoapods.org/specification.html#post_install">post_install</a> hook.
  But "<em>pod spec lint</em>" says that this hook is deprecated:</p>
  
  <pre><code>- WARN  | [iOS] The post install hook of the specification DSL has been deprecated, use the `resource_bundles` or the `prepare_command` attributes.
  </code></pre>
  
  <p>I have no idea how I can replace post_install hook with *resource_bundles* or *prepare_command*.
  Who knows any other approach to solve my problem? Is it possible?</p>
  
  <p>And another problem is how to modify Xcode project to add build phase, but it is actual only when "post_hook problem" is solved.</p>