* 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.

Still needs a ton of work to emulate argument handling, shell selection,
and output emulation in every case. Unsurprisingly, Ansible documents
none of this.

Sending just via .call_async() would previously crash the child, now it
generates CallError like intended.

Python 2.4 does not support explicit relative imports. They were added
at Python 2.5, along with `from __future__ import absolute_import`.

On 2.x this will mean the import is first (implicitly) tried relative,
but on 3.x it will always be tried absolute.

Fixes #92

Hopefully these are correct, and clearerabout the
behaviour/pre-conditions of these methods.

Not sure how they got there, but Django doesn't use Jinja without
explicit configuration.

This matches a tweak made in 8765f761c995c31c9bff6287d881f0e9b5d0cfc5