Update for bugfix and new features

Change log:
  bugfix:
    change all methods in class Bitcoin static (fixed: 'PHP Fatal error:  Using $this when not in object context in /Users/sskaje/Work/bitcoin/bitcoin-php/src/bitcoin.inc on line 215')

  new features:
    add support for conversion between private key and WIF; 

  other improvements
    add new BitcoinException; 
    new checksum() for common usage; 
    dos2unix; 
    update unit test script for new features.