Fix thread displaying inside the LMS and external site

This fix a lot of issues I had when displaying the xblock discussion in an external site using jquery-xblock. We can not visualize properly a thread discussion and also a subthread. I've added a TODO mark in all place I modified something so we can review later when finalizing the official xblock.

What are my priorities at the moment:

* Fix the url handling so we can remove my current hack. (See Discussion.Utils.urlFor). We need to tell the xblock javascript the LMS url somehow. Be aware of currently, there are only javascript files and they will be replace by the coffescript ones as soon as possible. @FiloSottile is supposed to check this, but if any of us find something useful.. we should advise here. The discussion module and those URLs endpoint are not directly done to any xblock handlers, but to the django_comments application.... so it is a little bit than xblock-mentoring.

* We can't post anything at the moment, neither vote. The error is: 2014-05-07 18:00:40,820 WARNING 27323 [django.request] csrf.py:184 - Forbidden (CSRF token missing or incorrect.). Again, we might find useful stuff in xblock-mentoring. 


BTW, this is how I generated our discussion.css: 
```
sass --cache-location /tmp/sass-cache --load-path discussion_forum/static/scss/bourbon/  --update -E utf-8  discussion_forum/static/scss/ && cp discussion_forum/static/scss/discussion.css discussion_forum/public/css/
```
(will be compressed and automatic later)

@FiloSottile checkout from this branch for your work, we'll rebase when merged.

@antoviaque @FiloSottile @jimabramson (and gprice), feel free to review the current work and contribute! :)