AWSServiceConfiguration not copying maxRetryCount

In the copyWithZone function the the AWSServiceConfiguration only
copies the region type and credentials provider but not the retry
count. This causes the copy to always have a retry count of 0.
Retry when network times out

In the case of a network timeout error, the nsurlsessiontask delegate
returns an error but not a response. The retry checking logic checks
the class of the response, which will fail if the response is nil.
This causes network timeouts to never be retried.
Merge pull request #49 from brsunter/master

Retry on Network Timeout
Version 2 of the AWS Mobile SDK for iOS 2.0.7.
Version 2 of the AWS Mobile SDK for iOS 2.0.8.
Updated README.md to use AWSCognitoCredentialsProvider as a default credentials provider.