Default logger to avoid warning when uninitialized in client code
Rework Pythran toolchain, a bit more modularity

This change is driven by JIT on-going implementation
Pythran is JIT enabled :)

This first version is somehow limited. No call to user function is
allowed in a jitted-function.
There is also no way to specify a spec for ahead-of-time compilation.

However the infrastructure is there and I expect we can add these
feature later.