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.
It's not only about implementation. Glacier is not really a BlobStore, so we need a way to document the mappings between the BlobStore methods and the provider methods. The user will know then what can they expect when calling the BlobStore method. I thought this would be the best place to document it.

What do you think?