Merged AST branch back into trunk. Most of this code was written by Marcin Kurczych for his Google Summer of Code 2008 project. The merge of this branch means that Genshi now uses the native `_ast` module on Python >= 2.5, and an emulation thereof on Python 2.4. This replaces the usage of the `compiler` package, which was deprecated in Python 2.6 and removed in Python 3.0. Another effect is that Genshi now runs on Google AppEngine (although performance is bad due to the lack of template caching).