JCLOUDS-457: Add blobExists and blobMetadata

Now the BlobStore abstraction supports the blobExists and the
blobMetadata operations.
> just a matter of style

From my perspective, just a matter of style. See e.g. [here](https://github.com/jclouds/jclouds/blob/master/apis/s3/src/main/java/org/jclouds/s3/blobstore/S3BlobStore.java#L130). But I think it's more important that we're consistent in _this_ class, so also OK to leave as-is.