Closes #103.

Ansible's PluginLoader makes up bullshit when it imports a module
(mostly because it has to make up something), therefore we ended up with
duplicate copies of ansible_mitogen loaded: one under
ansible.plugins.*.mitogen, and one under the canonical namespace.

Which broke isinstance().

Need a more general solution to littering the code with this crap.

Running in a thread to begin with, but this must change.

* Unused imports
* Undefined names in helpers.py
* Copyright header wrapping

It's used at least by the copy module, even though the result is still
mostly a no-op. _remote_chmod() doesn't accept octal mode, it accepts
symbolic mode. So implement a symbolic parser in helpers.py.

Fixes #90