list and deploy nodes with a certain tag

With this branch two new commands are added:

```
LittleChef: Configuration Management using Chef Solo

Available commands:

[...]
    list_nodes_with_tag     Show all nodes which have assigned a given tag
    nodes_with_tag          Sets a list of nodes that have the given tag assigned and calls node()
[...]
```

The idea is to be able list and deploy nodes with a certain tag, similar to how the `nodes_with_role` selector is working.

Sample calls would be like

```
% fix list_nodes_with_tag:boo
[...]
% fix nodes_with_tag:baz
[...]
```

Also an additional commandline switch called `--include-guests` is introduced. This switch is useful if you have the convention that all tags for the host are virtually also tags for the host's guests. In this case `fix` is not only applied on the host with a tag, but also on all of the host's guests.

I don't consider this branch done yet, but would like to get some input on the general concept first (esp. from @tobami). Missing bits are:

* documentation in README
* missing tests for the new functionallity (blocked by already failing tests in master #120)

Any initial comments?