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
Why not use Longs.toByteArray like you did in your answer? http://stackoverflow.com/a/25111119/624900