Replace old numpy macros with their new versions

Using the replace_old_macros.sed script distributed
with numpy.
Fix deprecated api usage in _png.cpp
Fix deprecated api usage in _image.cpp
Fix deprecated api usage in agg_py_path_iterator.h
Fix deprecated api usage in _path.cpp
Fix deprecated api usage in cntr.c
Fix deprecated api usage in _delaunay.cpp
Fix deprecated api usage in _backend_agg.cpp
Numpy 1.6 compatibility in _delaunay.cpp
Test with numpy 1.6 on Travis

Couple it with Python 2.6, since those are older versions of
the dependencies.
Fix C++ism in C file
Turn numpy deprecation warnings into errors

By defining a macro in setupext.py
Fix deprecated api usage in _macosx.m
Remove extra Py_XDECREF calls

Thanks to @mdehoon for catching these!
Merge pull request #3547 from jkseppan/numpy-deprecation

Conflicts:
	src/_path.cpp

Conflicted with bug fix for picking zero length path collection 5e2b03c1
PR #3592