disqus template context processor.

DISQUS shortname is handover to template context
from context processor.
js_body_bottom template block.
DISQUS comments count script attached.

For:
1. posts.html
2. tag.html
disqus script template block.
DISQUS comment links.

Links for:
1. post.html
2. posts.html
3. tag.html

Additionaly:
1. disqus.html included in post.html

This commit closes #69.