ATMEGA8U2 symbol added to atmel.lib

I've added a symbol for ATMEGA8U2 in atmel.lib. Please review and let me know if I have to make any changes.

The link to the ATMEGA8U2 datasheet is at http://www.atmel.com/Images/doc7799.pdf

When saving the library, the library editor replaced every
```
F3 "" 0 0 60 H V C CNN
```
line with
```
F3 "~" 0 0 60 H V C CNN
```
Is this fine, or do I change them all back?