Issues/2252 better embeds

Adds better support for embedded content into the iOS reader.  Iframes are now loaded in a webview that is embedded in the text view, as are video and object embeds.  DTCoreText doesn't have support for audio and embed tags so these remain to be added at a future date. 

Fixes #2252
Fixes #2212  
Is it there something missing to implement here? (Otherwise you can use: `// MARK`)