Merge pull request #141 from serge-sans-paille/always-raising

Always raising
Merge pull request #144 from serge-sans-paille/builtin-constructor

Builtin constructor
Improve None implementation and fix a few bugs

* the data field is now private
* typing when manipulating None<long> is done right (no implicit
  conversion)
Fix bad gsliced_ndarray to_index computation

* The computation was incorrect when the gslice was partial
* Force the use of the proper assignment operator, even if it seems
  redundant
Fix bug when raising uncaught exception & Add exception test cases

* Most C++ exceptions appear in Python as a RuntimeError.
  This bug is fixed by registering additional exception translators.
* However, there are 2 Exceptions can not be registered on Linux:
  WindowsError, VMSError
* Changed run_test for comparing the exception type raised in python
  and the type raised in pythran.