Add a new getSourcePath() method on the AssetInterface

Refactoring - it was a common pattern to get both the sourceRoot and sourcePath,
check if both are set, concatenate them and take the dirname().
Add changelog notice (BC break for custom AssetInterface implementations)
Fix test assertion (seems the filter now returns even more compressed CSS?)
Merge remote-tracking branch 'mpdude/asset-source-path'
fixed changelog