Library fixes

Hello, 

I have made two small fixes to the lib to make it work with newer AVR gcc than what Arduino ships and to make the display bias parametrizable. My display is from Sparkfun and goes all black with the default bias of 0x4 but works fine with 0x1.

- Fix compilation error (PROGMEM needs to be const with newer gcc)
- Added bias to the arguments of begin(), my display goes all black with 0x4 after a while.
  It works with 0x1. Some people even use 0x14, 0x15. It obviously depends on display.