Fixing macro.py's ability to render Python non-literals.

[benbdeitch]

This pull request alters the way in which Infogami renders macros.

Before, the macros were processed by creating a template to parse them through web.py's template system. Instead, it now functions through using the ast.parse method, as well as limiting the potential arguments and keyword arguments of the functions exclusively to Python literals.

[cdrini]

Lgtm! We patch deployed this one since it has security implications.