Add unique index for username and email to tl_member

It is currently possible to register with an already existing username and/or email if you add a 4-byte UTF-8 character to the end of the username/email.

![contao-register](https://cloud.githubusercontent.com/assets/367169/3615431/9572c080-0dc8-11e4-838f-ac5dbb917900.png)

Adding an unique index to the username and email columns would fix this issue. But I don't know if changing the columns to allow NULL values has backwards compatibility issues.