only write 8-byte data descr fields if needed
only write 8-byte data descr fields if needed

 * The original_size and compressed_size fields of the data descriptor
   should be 4 bytes, and only increased to 8 if the specific value is
   too small to be contained.

 * Increase our created_with, extract_requires version num.

Johnetordoff/waterbutler,TomBaxter/waterbutler,felliott/waterbutler,CenterForOpenScience/waterbutler,rdhyee/waterbutler,RCOSDP/waterbutler