Explicitly set timezone for iso8601 timestamps
Add `upcoming` helper method to mark content that should only be displayed when in the future
Toggle upcoming visibility client-side
Allow upcoming to take a date argument, simplify JS
Merge branch 'client-side-upcoming'

Conflicts:
	source/stylesheets/application.css.scss.erb
	source/talks/2013/10-17-deploying-your-rails-app-to-heroku.md.erb
	source/talks/2013/10-17-metaprogramming-with-ruby.md.erb