feat(splitter): chunking projectid, vaultid, start_marker and end_marker

This patch allows every vaultid specific to a projectid to be chunked
with a start marker and end marker. This allows the manager to be
instantiated with a different start and end marker when calling the
validation functions.

Addresses Issue: https://github.com/rackerlabs/deuce-valere/issues/10
Instead of L81-L82, why not add the pair here?

```python
marker_set = (storage_id[0].split('_')[0], marker.split('_')[0])
```

Adding a 'yield' appropriately and you could have the generator here...

This might also take care of some of the use cases for you since 'marker' should be None at the end of the list, and it'll auto-split it against the size