feature: Merge snapshots by latest package versions

Hey @smira!
This patch adds the ability to merge mirror snapshots with the latest version of each package (fixes #38). The command switch I used is `aptly snapshot merge -latest ...`. If the `-latest` flag is not provided, functionality is unchanged.

The `FilterLatestRefs` function should be reusable in case we need to do latest package filtering elsewhere at some point.

I looked for a place to update the [aptly.info](http://aptly.info) source but didn't find it, let me know if I am missing it somewhere and I'll be happy to update that as well.

Let me know what you think. Thanks!