Use wp_enqueue_scripts to enqueue Font Awesome

Allows it to be dequeued more easily when wp_dequeue_style is also
called on wp_enqueue_scripts
Allow the path to Font Awesome to be filtered and namespace the handle

Makes it easier for the user to use a different source for Font Awesome,
or to remove it entirely using __return_false. Also reduces the
potential for conflict with other instances of Font Awesome in plugins
or themes.
Fix filter and handle to for consistent naming
Allow for a custom list of post types that can support Tweetable Text
Update docblock
Don't include "via" in the Tweet if one isn't available

Also, build the eventual href inside makeTweetable, not inside the
tweet.php template, to keep the new logic out of the template
Allow parent and child themes to include templates
Construct the via= alongside the alt text and hashtag
Merge branch 'fallback-if-via-missing'

* fallback-if-via-missing:
  Construct the via= alongside the alt text and hashtag