vagga-docker should read global settings as well as vagga for linux

[a-urth]

Currently vagga-docker doesn't read global settings from following files
$HOME/.config/vagga/settings.yaml
$HOME/.vagga/settings.yaml
$HOME/.vagga.yaml

This is especially a problem due to some issues with fresh ubuntu installation (tailhook/vagga#263)

Version of vagga-docker - latest
Version of docker for mac - 1.12.0-rc2-beta17

[tailhook]

Mirror settings along with some other things work since 14781d0.

Known thing that doesn't work yet is external-volumes (ping me if you need this in the short term, I'm not sure I'll implement them before the merge otherwise);

[a-urth]

pathlib.Path.home() is python 3.5 only so readme should be updated accordingly

[tailhook]

Fixed to work with python3.4. I don't know if it matters much, but it was easy.