Option to change genre for song and write into ID3 tag

Another small enhancement which I very often use in my own ampache version.
I have many songs with empty genre tag in ID
With ampache on my mobile phone I can tag songs anyplace and anytime.
For this I added this "change genre in ID3 tag" which I now made optional
Would love to see this in your main branch

TNX
![change id3 tag](https://cloud.githubusercontent.com/assets/6050402/3279044/76ae0b0c-f3e5-11e3-899d-db18b5b4e600.jpg)