Adding support for the "Date added field"

Related to #128. This isn't yet finished. Just a draft of how we could implement this. I want to discuss it so we could find the appropriate way.

I'll try to explain some of the work I've done:
* I began storing the time (```time.time()```) but then I changed it because it's not user-friendly to set this information (if the user wished to change it after the import).
* I wanted to separate the items fields from the album; I think the behavior is that if a track is later added, it should have a different date from the rest of the album/tracks
* In the ```Library.add_album()``` I changed how the album is added to the database. Before, it'd only set the ```ALBUM_KEYS_ITEM``` and the rest of the fields would be ```None```. I just changed so it'd also set the "date added" fields before persisting it.

I'd like some input on this beginning so we could continue more easily implement this broadly through beets.

Thanks!