internal/blobstore, internal/multihash: new packages
internal/blobstore: new package

	This currently layers on top of juju/blobstore,
		but is designed so that it is easy to swap out
			that backend
internal/multihash: add a test
internal/blobstore: more understandable comment
add blobstore dependency
add new dependencies
Merge pull request #25 from rogpeppe/008-blobstore

internal/blobstore: new package

This implements the API that we need for storing
blobs, layered onto juju/blobstore.