refactors user's registration

Simplifies process and fields: 

* In: username
* Out: first_name, last_name, phone, nickname, use_nickname?

Closes #213

db/migrate/20150821130019_changes_user_registrations_fields.rb

@@ -0,0 +1,10 @@
+class ChangesUserRegistrationsFields < ActiveRecord::Migration
+  def change
+    add_column :users, :username, :string
+
+    remove_column :users, :first_name, :string
+    remove_column :users, :last_name, :string
+    remove_column :users, :nickname, :string
+    remove_column :users, :use_nickname, :boolean, default: false, null: false
+  end
+end