Rebased onto 1.1.x
Merge pull request #442 from mpdude/fix-clear-filters-on-collection-1.1.x

AssetCollection::clearFilters() does not work once the collection has been iterated (1.1.x)
fixed detection of deep mtime on collections
Removed '@' symbols because was throwing an error the @import was not included.
Merge branch '1.1.x'

Conflicts:
	src/Assetic/Factory/LazyAssetManager.php