Added AST3to2 stage to the pipeline

This transformation converts Python 3 AST to Python 2 AST (right now, only
the difference in function arguments are transformed). This enables current
Python 2 oriented Numba to optimize and compile the AST correctly.
Proper module definition of ufunc and gufunc in vectorize
Fixed test
Create py2only package for tests containing Python 3 incompatible syntax
Hack to remove unwanted warnings in Python 3

In Python 3 flow.is_tracked is not working properly. This should be improved.
Fixed tests for python 3
Added visit_Index to AST3to2
Changed tests for Python 3 compatibility
Added recurse co_consts in Python 3

In Python 3, variable names contained in generators are
kept in co_varnames within the generator which in turn is
contained within co_consts
In Python 3, __code__ should be used instead of func_code
Changed tabs to spaces
Changed to Python 3 syntax
Refactored the reporting in order to have the same semantics in Py 2 and 3
Fixed merge conflicts
Merge branch '_devel' of https://github.com/hgrecco/numba into devel

Conflicts:
	.travis.yml
	numba/pipeline.py