Add support for SNS message attributes.

See http://docs.aws.amazon.com/sns/latest/dg/SNSMessageAttributes.html

Fixes boto#2339
SNS message attributes fixes after actually testing with AWS.

Also make tests pass under python 2.6.
Fix key order in new SNS test
Merge branch 'rectalogic-sns-message-attributes' into develop. Fixes #2360.