Simplify API for Non-Perl

[gflohr]

The API is too complicated for languages other than Perl. In particular:

• Remove the necessity to call methods, whenever possible. That implies to pass the Site object again to most methods (had been removed recently).
• Prefer array references over hashes. For example the assets property of the Site object should be a an array reference.
• Simple tie mechanism for stderr so that it can be redirected to the logger.