Ec2Signer class should support AWS v3/4 signatures

Currently the Ec2Signer class only supports SignatureVersion 0/1/2, and AWS now have versions going up to v4 (introduced last year, and the boto client library now uses it by default for many services).

v3 looks to be a variation on v2 which is only used by Route53 and ses so probably not high priority (may as well do it though if it's easy), however the v4 is required to retain compatibility with current AWS client libraries/tools (e.g required to allow heat and nova to work with the latest python-boto which will be picked up by the distros in due course)