Copy, List, and Show commands

Bit of a multi-feature PR this one, I'm afraid, sorry. It was a series of things we needed for project that just came along all at once. It is:

* A `copy` command that copies package metadata from one component to another, pointing to the same package in the pool. We found we wanted to "promote" a package from, say, an unstable to stable component, and this operation is the most efficient way to do that.
* A `list` command that simply lists out packages and their versions in a component. The "long" option returns essentially the full metadata.
* A `show` command that shows one package, returning a helpful error code if it isn't found. Along with the "quiet" flag, useful for checking for existence in scripts.

I've also updated the `aws-sdk` gem, and modified the control file parser so that we can gather fields not explicitly looked for by the code. This is so that extra non-standard fields are returned by the `list` command.

All of this (along with the rest of the deb-s3 commands) has been in use by us over the past month and has been stable.