Retrieve customer profile by username/password

* Get Customer by providing login/password credentials
* @param username - customer login
* @param password - customer password
* @param onSuccess - invoked if customer with provided credentials was not found
* @param onError - invoked if exception occurred
* @return future with <CustomerInfo> result
public Future<CustomerInfo> getCustomerInfo(String username, String password, Callback<CustomerInfo> onSuccess, Callback<MobileSDKException> onError);
Calling this function It is equivalent to a make a login. The success of this function opens a session with the customer returned. To close the session use logout() method.