[Colleen, Drew] Custom tags

https://www.pivotaltracker.com/story/show/47110619

Biggest part of this pull is upgrading many of the dummy app's dependencies, e.g. rspec, capybara, and what not.

The actual feature is simply exposing a :tag option to the tandem_content_tag helper, so developers can specify a custom tag instead of having every one wrapped in a div.