Additional denominations support

Support denominations up to $100.
File name format changed to BTC_XXX_YYY.btc where X is the denomination (1, 5, 10, 20, 50, 100) and Y is the same BTCNumber used before. This allows the operator to fill the SD card with sets of different QR codes for each denomination. 
More debug statements for Serial Monitor. (Really helped in troubleshooting my terrible soldering skills).