Started work on update feature. Made functions for get_thread.

The get_thread function was too long, two function calls was added
to it, these functions contain the relevant code for retrieval of
thread information and post information. These functions are:
get_thread_information and get_post_information

I'm starting to see the strength of OOP here, because if I had done
it like that I could more easily save information in class variables
instead of passing around local variables.

The update feature is not completely thought through yet, but I
imagine it will work something like this:

1) See that the update argument is set, this sets the args.update
   value to True instead of False. We pass this to the get_thread
   function and move on to step 2.
2) We still retrieve the basic thread page and data information
   but then we check if the update function is set. If it is, we
   look up the previously written file in the output directory
   and open it.

   NOTE: The file_title was previous set in the create_xml
   function, but it has now been moved to the data dictionary
   and is created during call to get_thread_information.

   Moving on, we look up what page we need to continue from using
   the post_thread_page attribute set in the last post element in
   the xml file. We also look up the post_id attribute from the
   last post element. This allows us to move onto step 3.
3) Load the appropriate page and look at elements below the last
   post we added to the xml, and then just keep looking for more
   post elements to add to the xml until we run out of posts.
4) Now this post information should be saved in a list called
   post_dicts. Now the question is how do we modify the old xml
   file with the new posts. The obvious way to do it is to take
   the lxml.etree object we've saved previously, append the new
   posts to the posts element, and then just save it. This would
   require some changes however, such as splitting the create_xml
   function into multiple so we can access the code that creates
   post elements.

And that should be it. Seriously consider making this OOP instead.