Simlify importing ElementTree

As we no longer support older python versions, importing
ElementTree is much simpler. The documented way for
extensions to use etree still remains the same.
Now using universal code for Python 2 & 3.

The most notable changes are the use of unicode_literals
and absolute_imports. Actually, absolute_imports was the
biggest deal as it gives us relative imports. For the first
time extensions import markdown relative to themselves.
This allows other packages to embed the markdown lib in a
subdir of their project and still be able to use our
extensions.
Merge branch '2and3'