CarrierWave terminology is a storage "provider", not "engine".
"storage_path" is now "identifier"; cosmetics.
Bump Guard RSpec and let it pull in rspec >= 2.12.0, which we need to be
able to use `.and_call_original` in the specs.

Remove guard-cucumber as it appears to cause dependency resolution
problems; then, remove cucumber features from the Guardfile.

Commit the Gemfile.lock, update the .gitignore to reflect this.
Move ActiveRecordFile to a dedicated file.
Use the StorageProvider to set File#url; improve specs.

Although a file has a URL, it's not a property of the file itself.  (For
example, it should not be stored in the database, as then all records
would require rewriting if routing conventions change.)

The storage provider is well positioned to interrogate the routing layer
and decorate the file with a corresponding URL.  We will support Rails
routing conventions by default, but permit the user to override.

Improve the specs: configure the spec Uploader with our storage provider
to ensure correct method proxying; refactor.