title: Modify all HTTP traffic on an OS X
body: <p>Imagine an application that while running injects a "Hello world" message on every web page you visit. </p>
  
  <p>How do I monitor and modify all HTTP traffic happening on an OS X machine?</p>
  
  <p>Monitoring and modifying all browser traffic would be good enough as well.</p>
  
  <p>Solution can be language agnostic; Python would be awesome.</p>