Option to use iframe or not

This basically addresses https://github.com/quilljs/quill/issues/179.

Two things are pending for this PR to be ready:

1. We need to figure out what to do with the default styles. I think a css file would be better than creating style tags, especially when you don't want to use an iframe, and those styles will conflict with the ones in your page.
Where I'm using this, I'm making the `addStyles` method a noop, and moved all the defaults to a .css file which I can then customize or override.
2. I can't find a way to run the e2e tests. When I run `grunt test:e2e`, the tests start to run but I get a `NoSuchFrameError` when protractor attempts to switch between frames. This is also happening to me in `develop` branch. Any ideas?