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

Found due to a LGTM warning about unused loop variable (related). As far
as I can tell the callback was sending fullname multiple times. KeyError
check added because I found NestedTest failed - mitogen.parent had
mitogen as one of it's related, and mitogen was not in the cache.

Refs #61

Since the for loops don't contain any break statements the StreamErrors
will always be raised when the loop completes without the method
resturning.

See https://lgtm.com/rules/5980098/

Refs #61

Refs #61

Both these addtions are to address warnings in
https://lgtm.com/projects/g/dw/mitogen/alerts/?mode=list. Namely that if
a class defines an equality method then it should also define an
inequality and a hash method.

Refs #61

This seemed a reasonable streamlining, but I'm happy to be overruled.

This documents the existing behaviour, which may not be the intended.