Simplify Type inference

To the price of a small regression (see new skipped test), compilation
times are now reasonable with gcc.

The key is to *not* perform two typing passes but only one.

Former behavior is still available through the config file.
Full clang compatibility

After a few changes in the core::list code, pythonic is now compatible
with clang.

Most issues were related to implicit casts leading to ambiguous
conversions...

pythran/tests/cases/quicksort.py led to the return of uninitialized
value,but their is already an issue opened for this case.
Improve extended slices performances

Use an internal buffer to cache array indexing.
Update TODO list
IPython integration

It is now possible to use the magicfunction %%pythran to use pythran
from an IPython shell.