Add 'key change' to test
Panic on error saving blobs
Optimize repository layout

Use subdirectoriers for Data and Tree objects, e.g. store blob
`aec070645fe53ee3b3763059376134f058cc337247c978add178b6ccdfb0019f`
at
```
ac/c0/aec070645fe53ee3b3763059376134f058cc337247c978add178b6ccdfb0019f
```

Issue #58
Correctly deal with empty files