Add Sigv4/s3 presigner

This adds two new signers that implement the sigv4 query auth (i.e presigned urls): 

http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-query-string-auth.html


Part of this change was consolidating the signer modules into a single test module.  The actual new unit tests for this start at: https://github.com/jamesls/botocore/compare/presign?expand=1#diff-ba8c31bd2a7fcaa0fa779dc3b770f8aaR127


cc @danielgtaylor