Caret iframe support / refactor default document occurancies

Not ready to merge yet, i'm opening this PR for a discussion. We started it [here](https://github.com/alohaeditor/Aloha-Editor/issues/1138#issuecomment-39535981).

At first i would like to know if this is an issue which needs to be fixed. Right now caret is not rendering in case of aloha being fired up from top document on an element which is in iframe. Is this a use case for aloha? Should we fix it?

While digging in it i found out that somewhere Aloha uses default document, and this is subject to change for sure. In the current state this PR contains some document refactoring, fixes iframe issue and brings in one more sandbox case with iframe.