Auto detection of the S3 bucket & dependency management

Bucket is now auto-detected. Sample run command

    AWS_ACCESS_KEY_ID=XXXXXXX AWS_SECRET_ACCESS_KEY="yyyyyyy" go run imagearmory.go --storage s3 --bucket justatestingbucket2

All dependencies are now management using [Godep](https://github.com/tools/godep). **Important** - `godep restore` will overwrite the local workspace versions of dependencies used by `imagearmory`.