Add support for adjusting text in the WebView to the preferred zoom scale on Android.

More robust iOS support is forthcoming, but currently works with iOS7
Dynamic Type Fonts by setting the following css on the body and using
relatively sized fonts sizing for child containers, for example:

body {
   font: -apple-system-body;
}

body > .app {
  font-family: 'HelveticaNeue-Light', 'HelveticaNeue', Helvetica,
Arial, sans-serif;
  font-size:0.75em;
  font-weight: 200;
}