Added paste api, messages-framework for 404s/500s.

New public paste api added. Retrieval only, no submits yet.
Using the url: /paste/api?id=<pasteid>, pastes can be retrieved
in JSON format.

Better error pages. Added messages framework to the mix.
Any view can raise a 404 with a relevant msg to be displayed.

responses.py, htmltools.py both cleaned again. Function defs
have been alphabetized for better manual-searching.

A few template tags have been moved or refactored because they
didn't make sense (blogger -> comments_button/readmore_box!)

More changes need to be made to clean this up. I wish I would've
known more about Django when I started this. I am paying for
my mistakes now.