Creating new common AWS V4Signer.

This extracts the dynamodb signing process into a new common AWS Signature Version 4 (http://docs.aws.amazon.com/general/latest/gr/signature-version-4.html) signer in the aws package that can be used for signing requests for any service that supports Signature Version 4. The new aws/sign_test.go file runs the Signature Version 4 Test Suite (http://docs.aws.amazon.com/general/latest/gr/signature-v4-test-suite.html).

Additionally, I've updated the dynamodb package to use this new common signer and removed the old dynamodb sign.go and sign_test.go files.