Password reset

1) add PasswordResetController
2) add views for PasswordResetController
3) add specs for PasswordResetController
4) add code for PasswordResetController
5) add validation on password length

Please approve.
This method could be:

```ruby
  def update
    return render :edit unless @user.update(password: params[:user][:password])
    @user.set_token_to_nil
    flash[:success] = "Your password has been reset. You can login in."
    redirect_to sign_in_url
    end
  end
```