Add patterns for AWS S3 & ELB access log

Add grok pattern to parse S3 access logs, http://docs.aws.amazon.com/AmazonS3/latest/dev/LogFormat.html, and ELB access logs, http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/access-log-collection.html.
This test fails because the pattern for object_size is "(?:%{INT:object_size:int}|-)". The grok lib invokes .to_i on nil, thus returning 0.