"Modify" for non-string values

Added the possibility to modify float, integer e bool values (using explicitly true/false), which closes #193.

The behavior for the bool values is still confusing for the user. For instance, if a field is set to ```true``` and then we change it to ```false```, we see:
```
Artist - Album - Song
  field: 1 -> False
```
which is weird, but not the issue here.

Also, I was a little uncertain of mapping the constructors directly in the ```(ALBUM|ITEM)_FIELDS```, which I think is actually better, because I couldn't track where should be ```str``` or ```unicode```. With some direction on this, I can maybe change this.

Thanks!