Use DocumentTaskQueue to create Documents

[jaytaph]

@Kiyoshika has written some code for the DocumentTask and DocumentTaskQueue. They are not really in use but they could form a nice base for outsourcing the actual documentbuilding inside the parser to an external object/process. This is what we see is happening in lots of browsers.

Do we want to change the current system so we use the taskqueue instead of creating documents directly in the parser?