Convert build to buildr.
Move org.springframework.build.aws.ivy/* up to top-level.
Convert to aws-java-sdk and be less chatty.

Previously, after doing an `s3ls` to get a list of paths, the
S3Repository would make another `s3cat` per path merely to get
the object details (size, last modified, etc.).

So now we cache the S3ObjectSummary and only go back to S3 when
Ivy asks for the file contents.