Add Support for S3 Presigned Operations

You can use the Aws::S3::Presigner class to create presigned URLs for S3
operations.
Update Docs and Use Options Hash for Presign Init

Update Aws::S3::Presigner#initialize to use an options hash instead of a
positioned argument for :client.

Add Documentation strings to presigner.
Make PresignHandler a Subclass of Presigner
Merge pull request #114 from aws/presigned

Add Support for S3 Presigned Operations