JCLOUDS-457: Small fixes and documentation

Now the BlobStore and the strategy implementations are annotated with @Singleton.

Added a small documentation to the blobstore classes.
>  I thought this would be the best place to document it.

I would be tempted to put this in the Javadoc for `GlacierBlobStore` itself? Something like "This implementation of BlobStore is based on Amazon Glacier. This means that A, B, C, D..."