Move plumbum.cmd hack into __init__.py

[agfor]

I found it a bit confusing that you could

from plumbum.cmd import some_command

but you couldn't

import plumbum.cmd

I went looking at __init__.py and found it wasn't there at all, as I would expect it to be.

This patch moves the module hack into __init__.py so it can be easily found, and gives it a name (in addition to adding it to sys.modules) so it can be directly imported.

It also simplifies the code slightly, removes the hardcoded use of plumbum as the package name so that plumbum can be used as a subpackage, and allows:

plumbum.cmd["some_command"]

as local does.

[tomerfiliba]

thanks - it's in