first version of a command to list all nodes with a certain tag

Conflicts:
	fix
	littlechef/__init__.py
	littlechef/runner.py
get_nodes_with_tag: ignore guests which are in a different chef env than
their hosts
implemented 'nodes_with_tag' command

Conflicts:
	littlechef/runner.py
Added 'nodes_with_tag' to the list of allowed commandes to execute the
configuration process

Conflicts:
	littlechef/runner.py
fixed docstring for 'list_nodes_with_tag'
Document new *_with_tag commands
nodes with tag returns guests which explicitly have a tag even if include_guests is False. Adds missing tests
Correct help text of include guests option
Merge pull request #193 from tobami/nodes_with_tags2

Nodes with tags2