Reduced duplicated code and created methods for (de)serialization with byte arrays.

There are no functional changes, but two new methods:

• byte[] serializeItemStackToByteArray(ItemStack stack) throws IOException
• ItemStack deserializeItemStackFromByteArray(byte[] input) throws IOException

Design part:
Up till now the serialize base64 implementation did two things (serialization and base64 encoding), they are now separated.

As it is now, you don't have to use a deprecated method or encode your byte[] to base64 only to let it be decoded again.

I'm not too sure about the naming, but it should be decent.