Support verification of signed requests and other special presigned request behavior

[kherock]

This includes full support for signature version 2 and partial support for version 4. This also supports specifing x-amz-meta-* headers via query parameters and overriding response headers via response-* query parameters.

The test suite is by no means comprehensive but should cover the most important errors typically encountered. There are a lot of edge cases that I've done my best to handle as accurately as possible.

For future reference, here's the list of the features currently not supported in the signature version 4 implementation:

• Verification of a provided signature (and calculating signing keys)
• Verification of content hash for signed payloads
• Validation of each component of an Authorization header
• Well-formedness check of Credential, SignedHeaders, and Signature parameters
• Validation of the date of a Credential parameter
• Enforcement of the SignedHeaders component