Cover art support

- implement database scheme
- search for embedded cover
- search for cover in track directory
- save covers in disk cache
- search for covers in disk cache
- minimal ui (display covers in lower left corner)
- handle existing users
- LRU cache
What about doing it:
```
1. stored path Path in DB  
2. embedded in file metadata
3. %track-file-base%.jpg in the track directory for %track-file-base%.mp3
4. %album%.jpg
...
```
instead of doing it:
```
1. stored path Path in DB  
2. %track-file-base%.jpg in the track directory for %track-file-base%.mp3
3. embedded in file metadata
4. %album%.jpg
...
```

All logic after **4** (including it) is related with searching into trackDirectory. In this way, doing **2** after **3** would bring some improvements in performance and code reading... Do you think that we should reorder it? or it's really essential doing **2** before **3**?

PS: of course, it's not a big deal... just an idea...