Adds twitter widget aside to Oscailte theme

The default octopress theme removed the "Latest Tweets" aside after
Twitter updated their API to v1.1 and removed support for unauthenticated
API calls. Twitter does, however, offer a widget that integrates well
with blogging platforms, including Octopress.

Jmac discusses integrating the twitter widget with Octopress  on his blog:
http://blog.jmac.org/blog/2013/03/30/putting-twitter-back-into-octopress/

This commit integrates Jmac's suggestion by:
* creating twitter.html in source/_includes/asides as discussed by Jmac
* adding a check for the site.social.twitter variable to display the aside
* utilizing the variables in _config.yml including:
  - site.social.twitter (string)
  - site.twitter_tweet_count (int)
  - site.twitter_show_profile_link (bool) (NEW)
  - site.twitter.widget_id (int) (NEW)
* adding a link button to the header, to match the style of other asides
  in Oscailte (e.g. Github)
* adding classes to section, h1, and ul to match the style of other asides
  in Oscailte

Users will have to add the NEW variables to _config.yml, which means creating
a new timeline widget on Twitter and obtaining a twitter widget id:
https://twitter.com/settings/widgets/new