Single code base for python 2 and 3, without using 2to3.

This is a minimal version, adding no dependency on six or
future modules.

It passes the basic tests, but 'pip install -e .' works only
on python 2.  A normal installation works on python 3.

I also added a .gitignore file taken from matplotlib and
lightly edited.
Merge pull request #2 from efiring/no_2to3

Single code base for python 2 and 3, without using 2to3.