Certificate XML encoding

[lucasperin]

There are two Certificate XML encoding functions in the Certificate class.
Both functions do the same thing, except that one converts the serial to decimal using the BigInteger class.

Since the serial can have (and usually has) length larger than 32 bits, it would be best if we always used BigInteger conversion. This approach handles both 32 to any bit precision for serial encoding gracefully, wehereas the approach with log/int does not.

Additionally, the serial should be encoded as HEX and not Decimal.

I propose using a deprecated tag to the integer solution and make it return a call to the BigInteger function. Also, changing the toDec() to toHex() should be a better solution to both.