Made scraping universal for all Tumblr blogs

DBC final presentations may have come and gone, but in some ways this felt like my final exam.

The version of this app that we presented worked like magic for many Tumblr blogs, but the fact that we were scraping HTML output and looking for specific tags left us at the mercy of the construction of any given user's Tumblr theme. Users whose posts featured a prominent <article> tag, for instance, were in great shape. Users whose posts did not were often left in the dust.

This pull request fixes the issue by taking better advantage of the Tumblr API. We're now displaying only the content we want to display, and the displays should work for all Tumblr users—regardless of the makeup of their themes. Doing this broke nearly every single test we had constructed, so 'fixing the fix' became a longer process than I would have liked.