MAINT: clean up convolution.template.h
MAINT: make comment style in header files portable.

See Numpy C style guide for details.
MAINT: clean up unused definitions in c_python.pxd
MAINT: clean up common.h, remove support for Python 2.4
MAINT: clean up convolution.template.c for comments and line length.
Merge branch 'comments' into master