How to add a custom field to devise in Rails 4 and customise the registration controller?

I am trying to add devise to the rails 4 app and i want to add new fields in registration and remove some existing field. I have changed my routes to

devise_for :users, :controllers => {:registrations => 'registrations'}

and i have included the following line in my application_controller.rb

class ApplicationController < ActionController::Base

  before_filter :configure_permitted_parameters, if: :devise_controller?

  def configure_permitted_parameters
      devise_parameter_sanitizer.for(:sign_up) { |u| u.permit(:username, :password) }
  end

and i have the following in my model file (user.rb)

class User < ActiveRecord::Base

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable

  attr_accessor :username
end

and my registration_controller.rb contains

class RegistrationsController < Devise::RegistrationsController
        skip_before_filter :require_no_authentication, :only => [ :new, :create, :cancel ]
        prepend_before_filter :authenticate_scope!, :only => [:edit, :update, :destroy]

        # GET /resource/sign_up
        def new
            super
        end

        # POST /users
        def create
            build_resource(sign_up_params)

            respond_to do |format|
                if resource.save
                    format.html { redirect_to profile_update_path, notice: 'User was successfully updated.' }
                else
                    format.html { render action: "new" }
                end
            end
        end

        protected

        def sign_up_params
            devise_parameter_sanitizer.sanitize(:sign_up)
        end

    end

the result is

Started POST "/users" for 127.0.0.1 at 2014-03-05 20:11:43 +0530
Processing by RegistrationsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"/Tu/QaH1gQgr73uND+fYcLzwer4yhaserghjNQxqazp=", "user"=>{"username"=>"testname", "password"=>"[FILTERED]"}, "commit"=>"Sign up"}
   (0.2ms)  BEGIN
  SQL (0.4ms)  INSERT INTO "users" ("created_at", "encrypted_password", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["created_at", Wed, 05 Mar 2014 14:41:43 UTC +00:00], ["encrypted_password", "$2a$10$9dLwOBN4qEc3Vgv8NiMVlOaOG.j4jbKNIEg1RPZPdohZYZsZQBY.."], ["updated_at", Wed, 05 Mar 2014 14:41:43 UTC +00:00]]
   (39.4ms)  COMMIT
Redirected to http://localhost:3000/profile
Completed 302 Found in 114ms (ActiveRecord: 40.0ms)

it does not save the username in the table. Please tell me where i went wrong. Thanks in advance!

UPDATE

The following is the list of fields i have in the table

id | email |                      encrypted_password                      | reset_password_token | reset_password_sent_at | remember_created_at | sign_in_count | current_sign_in_at | last_sign_in_at | current_sign_in_ip | last_sign_in_ip |         created_at         |         updated_at         | username | first_name | last_name |