Ensure sources.list exists but is empty (#19)

Salt uses the aptsources system library to get a list of repositories on
the host. Unfortunately if the /etc/apt/sources.list file is missing,
aptsources.sourceslist.SourcesList() (which is called by salt) logs a
warning, which is then displayed to the user running salt.

To avoid this warning when using 'remove_sources_list: true' in pillar
we can keep the /etc/apt/sources.list file, but just ensure it is empty.
This achieves the same thing, but does not log the warning or display it
to the user.

Add protocol and port in pillar repository example for 3rd party repo

When adding, gpg will fail with a "not able to connect" in some cases (most likely related to firewalling, but not investigated thoroughly): adding the specific protocol and port solves this issue.

Add a state to manage apt's preferences, with tests

Ignore .kitchen dir

Add defaults for preferences mgmt

Add some pillar examples for preferences mgmt

Add tests for preferences

Add entries in the README file

Missing map.jinja entry

Fix resource name to avoid conflicting ID

Add a state to manage repositories, with tests

Correct formula name

Fixed pillar.get calls

Resolve map variable conflict

Add state for unattended upgrades.

Change states to use short-dec style

ENH: Add apt.ppa for pkg:python-software-properties (add-apt-repository)

Add apt.transports.https and apt.transports.debtorrent

DOC: Use full state names; add RST formatting

Fix broken link

General changes to standardize formula format

Introduced apt directory to conform to saltstack-formulas structure.