Rewrite

A rewrite of this, using forkdb.

Will probably also rename this to `revisions` (since it's not really a level-* project anymore).

Each revision is saved in `[key, timestamp]` I'm using the `heads`-features from `forkdb` to assure atomicity (#1) - basically each `[key, timestamp]` can have multiple values associated with it. When we `get()` the revisions we iterate over all keys and do the merging then. It seem to be a bit slow for very large keysets, so I might need to spend some time thinking about how to go about that in the future. But hey, it works!