Clean up.

More OCD changes, nothing functional.
Correct readme URL's.
Don't attempt to generate OpenGraph tags on 404.
Improve OpenGraph and Twitter Card support.

By supporting OpenGraph Webcomic already mostly supported Twitter Card.
This commit includes various improvements that ensure both function
optimally.
Update Twitter Settings documentation.

The list of available tags has been relegated to the Beginner's Guide
to cut down on the vertical space. Includes notes on using Twitter
Cards.
Add webcomic-attachment count class to post_class.
Add webcomic_count template tag.

This tag should be useful for users that want to do different things
with their webcomics based on how many attachments the webcomic post
has.
Add webcomic_count shortcode.
Ensure media uploads are properly attached.
Correct media function call.
Make [data-webcomic-shortcuts] absolute.

The data-webcomic-shortcuts attribute–if assigned to an element on the
page–now forces Webcomic to only look for keyboard shortcut links
inside of the marked container.
Ensure Webcomic Media is updated.
Ensure uploads function when post content is disabled.
Correct spacing after period.
Add touch gesture support.

Includes updates and corrections to inline documentation. Closes #84.
Mention touch gesture support in feature improvements.
Clean up.
Add touch gesture support to the Dynamic Webcomic widget.