Python converter for std::set

- New converter in triqs/python_tools/converters
- Extended wrap_test
- Corrected order of includes in my_module_desc.py
Variadic generalization of boost::variant converter

- Had to use some MPL iterators, since boost::variant is based on MPL.
Merge pull request #154 from krivenko/new_converters

[converters] Two changes in Python converters