Add Crc32c.getValueAsBytes

Some callers want the byte array representation, e.g., for Google
Cloud Storage objects.  This method may help avoid confusion such as:

http://stackoverflow.com/questions/25096737/base64-encoding-of-crc32c-long-value
Merge pull request #2 from andrewgaul/get-value-as-bytes

Add Crc32c.getValueAsBytes