Fix assets minification and combination

- set correct namespace for CssMin (it's \CssMin)
- assets with urls starting with / are relative to DIR_BASE, not to
  DIR_APPLICATION
- to retrieve the class of an asset: let's use get_class, since
  concatenating AssetType + 'Asset' does not contain the class namespace
Merge pull request #213 from mlocati/fix-assets-minification-combination

Fix assets minification and combination