JCLOUDS-457: Add blobExists and blobMetadata

Now the BlobStore abstraction supports the blobExists and the
blobMetadata operations.
Again, if this is a jclouds-wide thing it also makes more sense to me changing it, I agree with @demobox that we should be consistent.

Any thoughts about this @andrewgaul? I copied the style from s3, azureblob and atmos in the past, and in many cases it wasn't right either.