README format changes
Test: Test sub creation, thin and fat
Start off subdue.sub module

This module contains code to be used by the driver of each sub.
Setup: include subdue.sub module in dist
Remove old exports from subdue.core
Introduce compatibility with python 2 and 3

I intend to keep subdue compatible with Python 2.7 and Python 3 from 3.2
onwards with the same codebase.

In this change, I introduce the use of the `six` module by Benjamin
Peterson, plus some of my own additions, in a core module called compat.
Make pypy compatible

The only incompatible bit so far was how I was finding the calling
script, by looking at the bottom of the stack. In pypy, it seems like
the interpreter is there, so I had to rule it out and keep going through
the stack until something different comes up.
Add more verbose error message on test failure
Handle the running of command containers

For now, just die when a command resolves to a container, but at least
detect that this has been the case.

In the future, running a container should show some help for the
container.
Document and improve Command class interface
Add option to check if command is eval command
Add env var handler

Will load the environment of the called command with a series of
variables through which subdue communicates with the commands it runs.
Test with tox!