"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.