Ensure we check for `URI` in management URI

Had an issue where the documentation had been removed and a client set
`storage_management_url` as a `String` which failed to respondto `#path`

This adds the documentation back to specify the contract expected and
now we raise if an invalid object is passed.
Merge pull request #16 from fog/clearup_storage_management_url

Ensure we check for `URI` in management URI