Key encoding

Currently it is not possible to use `ttl` with keys that are not utf8, as string concatenation is used for composing the new 'ttl' key. The result is a laconic:
```
      -
        key:   ÿttlÿ1371634468334ÿ[object Object]
        value: [object Object]
      -
        key:   ÿttlÿ[object Object]
        value: 1371634468334
```

We need to be able to encode a key to string.
In the end, we need:
 * accessing the current default encoding of the database.
 * accessing the utilities for enconding/decoding from LevelUp.

I am attaching a (failing) test.