Add addgroup, addclass, listgroups, listclasses tasks for nodes

Previously if a user wanted to use rake tasks to interrogate the dashboard
about which groups or classes a node belonged to, they would be out of luck.
Similarly, if a user wanted to add a class or group to a node without replacing
the already assigned classes/groups of a node it was not possible via only
rake. This commit adds a task in the node namespace to add a group or groups to
a node and to add a class or classes to a node. The other two tasks added here
list out the groups or classes already assigned to a node.
Fix descriptions of node tasks to match reality

The descriptions for the node:classes and node:groups tasks are misleading in
that they indicate that the classes and groups of a node can be edited or added
to using the task. The actual tasks will replace, and only replace, the classes
and groups for a node so this commit updates those descriptions to indicate
that.
Add get_node helper method to node.rake

As there was a large quantity of repeated code in the node rake tasks, this
commit adds a helper method to get a node from the name environment variable
and to exit with error if the node doesn't exist or can't be found.
Merge pull request #163 from haus/add_helper_tasks_to_node

Add helper tasks to node namespace