added a direct AwsInstanceDiscovery impl.  created constants out of some
heavily-reused strings, updated build.gradle to include aws sdk where
appropriate, moved eureka-client initialization to
EurekaInstanceDiscovery, updated comments in config.properties to
better-explain each property
added comments related to the AWS Credentials needed by the
AwsInstanceDiscovery implementation
udpated config.properties to use ConfigPropertyBasedDiscovery as the
default.  add more comments for this particular implementation
redo
Merge pull request #18 from cfregly/master

Cleaned up eureka/turbine configuration, added comments, added a new direct AwsInstanceDiscovery impl that uses the AWS API to retrieve instances for an ASG (aka cluster in Turbine terms)