Rework None implementation

Previous implementation had typing information spread over several files
(bad locality) and did not handle using member function on the none
type.
Add missing const qualifier for string::operator==
Use a CFG analysis to insert Return None in the missing places

* This fixes Issue #77
* The CFG does not take into account interprocedural exceptions
Make test_random more robust

Increase the number of sampling when testing random generator
implementation, to avoid false negative.

Execution time is still reasonable compared to compile time ;-)
Workaround clang++ list constructor bad disambiguification

clang++ is not capable of resolving against the right constructor for

    core::list<empty_list>({ proxy::list{}() })

so help it a little.
Merge pull request #128 from pbrunet/master

Improve numpy array + bugfixes

- Perf&features improvements for array slicing
- Handle case of empty string + sliced string export to python
- Indexed_array
- Add dtype and T attributes to numpy array
- Many tests from rosetta and for numpy
Fix bug in a corner case of list slicing

Has exhibited by the fannkuch.py test case
Fix bug in while: else: backend implementation
Merge pull request #131 from serge-sans-paille/few-bugfixes

Few bugfixes in list slicing and while/else C++ backend
Merge pull request #130 from serge-sans-paille/return-insertion

Return insertion
Add missing specialization for scalar option types
Generalize closure implementation

The `bind' call can now create closure of any size.
Merge pull request #133 from serge-sans-paille/closure

Generalize closure implementation
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