Add key-based authentication to management service

Intended to be the simplest auth possible: every time the service
starts, a random key is written to `/var/lib/mailinabox/api.key`. In
order to authenticate to the service, the client must pass the contents
of `api.key` in an HTTP basic auth header. In this way, users who do not
have read access to that file are not able to communicate with the
service.
Update mail tool to pass api key auth
Update setup scripts to auth against the API
Update documentation to use API auth

The updated instruction is not very user-friendly. I think the right
solution is to wrap the `/dns` commands in a `tools/dns.py` style
script, along the lines of `tools/mail.py`.
Merge remote-tracking branch 'upstream/master' into mgmt-auth

Conflicts:
	management/daemon.py
Remove API_KEY_FILE setting
Cleanup: remove env dependency
Merge pull request #74 from mkropat/mgmt-auth

Add authentication to mailinabox-daemon; resolves #67