add setosc example

Author: ladyada

examples/pwmtest/pwmtest.ino

@@ -31,6 +31,10 @@ void setup() {
   Serial.println("16 channel PWM test!");
 
   pwm.begin();
+  // In theory the internal oscillator is 25MHz but it really isn't
+  // that precise. You can 'calibrate' by tweaking this number till
+  // you get the frequency you're expecting!
+  pwm.setOscillatorFrequency(27000000);  // The int.osc. is closer to 27MHz
   pwm.setPWMFreq(1600);  // This is the maximum PWM frequency
 
   // if you want to really speed stuff up, you can go into 'fast 400khz I2C' mode