Fetching and Parsing Feeds

* Set dashboards#show as root_path for signed_in users
* Add Feedjira to Gemfile
* Add sources and feed_entries tables to database
* Add fetching and parsing methods to source and feed_entry models
* Show user.timeline on dashboards#show
* User can see sources#index
* User can add new sources
I would make content_feed return an empty array if you don't have any sources (or if those sources don't return anything). See my comment in the view to see why.