Added next release section to the changelog. [ci skip]
Organized release rake tasks. [ci skip]
Refactor of resource validators.
Added Aws::IAM::AccessKeyPair.
Updated Resource definition load paths from $ to @.

Was previously using "$" as a placeholder path expression to
return the searched data object. Now that the SDK uses
the `jmespath` gem, we can use the actual JMESPath expression
"@" which has the same meaning.

Updated all of the resource definitions and test to use the
"@" symbol.
Consolidated object version shapes for S3 ListObjectVersions.
Resolved an issue with parsing S3 object keys from #list_objects response.

Fixes #161