Remove all references to __user__

This was an ugly artifact introduced in the first days of pythran. Rely
on a proper member variable instead.
Harmonize user function and standard function combiner

Use the same interface and do not distinguish between the two during
ast.Call processing.
small rework of string::operator*
Rework slice to normalize slice writing and avoid code duplication
Remove compilation warning for truncate function
Remove compilation warning about numpy depreciate API
Merge pull request #134 from serge-sans-paille/closure

typing.py : code cleaning
Merge pull request #132 from pbrunet/slice

Rework slice to avoid redundancy in the code + warnings removed
Prevent boost python registration warning

Now check whether the converter is already registered in a more robust
way.
Fix generator backend

A variable was not correctly initialized in the default constructor.
Merge pull request #138 from serge-sans-paille/no-boost-python-warning

Prevent boost python registration warning
Merge pull request #139 from serge-sans-paille/generator-init

Fix generator backend