add custom attribute method

[sjoshi10]

I was trying to register using custom attributes but I wasn't able to. I modified the Cognito class by creating two methods that allows you to add base and custom attributes. add_custom_attributes method will take attributes and modify key so that you will be able to add custom attributes. With these methods, you don't have to pass attributes into Register method.

You can register by using these methods:

u.add_base_attributes(email='test@gmail.com')
u.add_custom_attributes(state='virginia', city='Centreville')
u.register('username', 'password')