Add authentication and User model

* Add User model
* Add SessionsController
* Add password authentication on User
* Request authentication for all actions except sign in
* Add some helpers for ApplicationController
* Update features to work with mandatory authentication

features/step_definitions/tracks_step.rb

@@ -16,6 +16,5 @@ end
 
 Then /^it should provide an audio stream for "([^"]*)"$/ do |name|
   page.should have_xpath "//audio[@src='#{stream_track_path(@track)}']"
-  get find('audio')[:src]
-  last_response.status.should == 200
+  visit find('audio')[:src]
 end