add mobile feature, thanks @vcuculo

features/mobile.feature

+@javascript
+Feature: Navigate mobile site
+
+  In order to navigate Diaspora*
+  As a mobile user
+  I want to show mobile site of Diaspora*
+
+  Scenario: Show mobile site
+    And I visit the mobile aspects page 
+    Then I should see "sign in"
+

features/step_definitions/mobile_steps.rb

+When /^I visit the mobile aspects page$/ do
+  visit('/aspects.mobile')
+end