adds public IP option to instances in VPC

allows you to pass :associate_public_ip_address to InstanceCollection#create,
and applies this value to the network interface options.
NOTE: in order for this to work, a couple of other options have to be moved
out of the base and into the network interface:
private_ip_address, subnet, and security_group_ids
Merge branch 'master' of https://github.com/aws/aws-sdk-ruby
delete the associate_public_ip_address option from create instance params whether it is true or false
Merge pull request #388 from elementaltech/master

add public IP address option for instances in VPC