SaltStack

Initial [SaltStack](http://docs.saltstack.com/index.html) entity driver implementation, based loosely on the existing chef-solo concepts.

The masterless configuration is working, and it is now possible to write entities that are installed using Salt formulas.  A basic `PostgreSqlNode` implementation using Salt is included, which is similar to the `PostgreSqlNodeChefImpl` example.

- The live test of `PostgreSqlNodeSaltImpl` fails currently, because the Salt formula used does not accept configuration properties for port numbers and so on.
- There is also an initial code framework for a Salt master entity.

These WIP issues will be addressed in a future pull request.