Add support for metadata

[3coma3]

The idea is to be able to manage metadata about snapshots. Maybe in some specific snapshot you had some data and you want to remember that fact, or classify it, or use some kind of organization of your snapshots. Here is a proposal for a framework to do that kind of stuff.

So you could have some fields like tags, description (those two would be good to start with) and do some basic CRUD and search operations on them. These operations could be combined, allowing new semantics at the snapshot repository level.

The fields could be stored in symlink/file structures under, say $base/.{fieldtype1, ..., fieldtypeN} or maybe in extended attributes. It should however be stored under the base of the snap repository using the filesystem and nothing else