Implement form validation on username input field

[TheTeaCat]

[TheTeaCat]

The last.fm signup form says:

Your username should be between 2 and 15 characters, begin with a letter and contain only letters, numbers, '_' or '-'.

...but in classic last.fm fashion, there exist users with 1 letter usernames!

Until I know more about the actual spec for a last.fm username, as little constrictive an approach seems sensible; I'll limit solely to excluding ampersands.