This commit restructures the core of charms.docker to follow the python best practice of import modules in the __init__.py file for a nicer UX

You can now import like so:

`from charms.docker import Compose`
or
`from charms.docker import DockerOpts`

This also brings with it a pretty hefty update to the Compose class, implementing methods and tests for the following methods:

 - start
- stop
- pull
- restart
- build
- scale

It also splits apart the private __run method into a separate module known as 'runner'.