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.

These test the current behviour, which may not be exactly the intended
behaviour. Refs #98

Farewell, shifty hacker dude! May your 0days be many and your
indictments be few

Some differences are eventually likely to become permanent, because the
existing behaviour is unforgiveable.