Graphic EQ Effect

Thiss PR adds an 8 band static equalizer based on Biquad filters. The implementation of the Biquad filter is based on @daschuer's iireqs branch.
This actually works, but can we be sure for all locales? 
We should change it to use snprintf(), it should follow the same rules for locale as strtod(). 
So you may give this a try:
```
char spec[32]; 
snprintf(spec, sizeof(spec), "LsBq/%.10f/%.10f", Q, dBgain); 
```
You have to deal a bit with precision and buffer size: 
http://stackoverflow.com/questions/16839658/printf-width-specificer-to-maintain-precision-of-floating-point-value