Support for EC2 Instance Profile instead of AccessKey/SecretKey and support for Key Prefixes so that multiple environments can share the same S3 bucket

[BenWolstencroft]

We host our sitefinity website on ec2 instances within amazon, these instances have an instance profile (and an IAM role associated with them), we'd like to use this role when accessing the S3 bucket for blob storage.

Also, we have multiple deployment environments (Testing, PreProd, Production), and would like these environments to have the same libraries configured (names etc) but with different content, within a single S3 bucket

I've submitted a pull request which adds these features: #9