API suggestions

[frank-rollpin]

Thanks for the great library! I really like that the emphasis on performance, this is very important to our project. However, I think its API can be improved a bit, so that its usage would be more in line with existing numerical types int and double. Here is what I have in mind:

• add fromByteArray constructor
• annotate all public methods with types of the return values
• provide a standard method for parsing, like int does:
  BigIntger parse(String source, radix, onError(String source) });
• add toDouble() method
• implement Comparable

Would you please consider making those changes?