Customize HTTP vendor prefix for Google Storage

Google storage uses/expects custom headers like meta data
and storage class to be prefixed with "x-goog" instead of
"x-amz". This enables use of object tagging in the Google
Storage provider.

Closes #356

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Fix link.
Add a note about retrieving object meta data in the Google Storage driver.