use blob names instead of hashes

We can't use content addressing with juju blob store,
as discussed ad nauseam online.
fixes after review
gofmt
Merge pull request #73 from rogpeppe/034-use-names-not-hashes

use blob names instead of hashes

We can't use content addressing with juju blob store,
as discussed ad nauseam online.

This is a redo of https://github.com/juju/charmstore/pull/71,
which had accidentally pulled in some code from another
branch.