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  
You should be able, in these enums, to do something like this:
```
autoresizingMask = .FlexibleWidth | .FlexibleHeight
contentMode = .ScaleAspectFit
```