[WIP] asset dependencies

This is basically what I'm thinking for making Assetic aware of asset dependencies. Each filter can optionally implement `getChildren($asset, $factory)` and return an array of assets that the supplied asset depends on.

The `LazyAssetManager` has the factory, so I've added a `getLastModified($asset)` method there, which considers the mtime of each dependency as well as the root asset.

### Todo

 - [ ] implement `getChildren()` on applicable filters

Thoughts?