Added gitignore for AVR C projects

Ignores the files generated when building an AVR C project with the 'standard' WinAVR Makefile template ([http://todbot.com/projects/smart-leds/softpwm_t13/Makefile](http://todbot.com/projects/smart-leds/softpwm_t13/Makefile)).

This can really be used with any generic C Makefile though, as it ignores most of the files that could be built in an AVR project:
- *.dep (make dependencies directory)
- *.hex (AVR hex files)
- *.obj, *.o (Object files)
- *.eep (eeprom data file)
- *.map (linker info)
- other AVR build files