Forgot to add test for BaseAggregateAsset::eachLeaf().
createMockFileAsset -> createStubFileAsset.

Also make the type param default to 'css' for brevity where we don't
care.
Introduce BasicAssetCollection, parent to all collections.

Rather a big refactor.
Clean up a bunch more stuff pursuant to the BasicAssetCollection change.
Get BasicAssetCollection::doReplace() to 100% coverage.
Copy over Assetic's AssetInterface methods into BaseAggregateAsset.
BaseAggregateAsset -> AssetAggregate; get rid of Css/Js aggregates.