Removed remaining initialize() method.
Updated the names of the concrete asset classes to make them more
consistent with each other.
Updated the constructors for all concrete Asset classes to take the
asset file directly, since we no longer need to cater for non file based
assets now that asset creation isn't managed centrally.
Updated the ThirdpartySourceModule constructor to not even require an
asset file, since there isn't a single asset file in this case.
Removed remaining empty constructors which are no longer needed now that
asset file construction is no longer centrally co-ordinated.
Updated the concrete Asset class constructors to match the
AssetPlugin.createAsset() interface method.
Merge pull request #633 from BladeRunnerJS/asset-constructor-cleanup-continued

Asset constructor cleanup continued