title: include files recursively in Cocoapods podspec
body: <p>I want to create a local podspec that is based on some private code. I can't seem to use the 'source' attribute, as that is not working. I can use the 'source_files' attribute, but it does not include files recursively. So with a directory that looks like this</p>
  
  <pre><code>Library
    /src
      /Core
      /Audio
      /Graphics
  </code></pre>
  
  <p>And my podspec looks like this:</p>
  
  <pre><code>Pod::Spec.new do |s|
    ...
    s.source = 'src' # this does not work.
    s.source_files = 'src' # this only includes the files in src, and not in any of the Core, Audio or Graphics folders.
  </code></pre>
  
  <p>I kind of want to specify a '-r' flag. I have tried using wildcards but no luck.</p>