Compatibility with modern versions of Django + misc improvements

First, thanks for making this - I find it very useful.

Here are the changes I made:
* Updated admin media/static files usage to be compatible with modern versions of Django (I'm on 1.5)
* Show HTML preview in an iframe so that the admin CSS affect its styles
* New "view in separate window" link for HTML preview (e.g. to test responsive styles)
* Removed the add button in the admin change list (there is no need/ability to manually add)
* Removed symlink instruction from README (not necessary anymore)
* Added .gitignore file

Please check it out and merge if it's all good  :+1: