Modernize to support Python 2 and 3
Remove backwards-compatibility code.

Now that we require minimum Python version 2.6.1, we can rely on
CalledProcessError and check_call() being present in the subprocess
module.
Document the Python interpreter versions that are supported.
Merge branch 'python3'

Allow git-imerge to run under Python 3.x, version 3.3 and greater.  It
should continue to work under Python 2.x, version 2.6.1 or later.
Document the commands "git imerge merge" and "git-imerge rebase".

Restructure README.txt and the in-program help to emphasize these new
commands as simple ways to start an incremental merge.  Also simplify
the in-program help considerably.