Add nodes_with_status method to v3 api

This method returns queried nodes and adds information on their
status and the events of the latest report. The status is saved
as string and can have the following values:

 - unreported
 - changed
 - unchanged
 - failed

A node gets unreported by default after two hours without report.
This can be changed with the `unreported` parameter of the
`nodes_with_status` method.

Node objects now have new instance variables:

 - status: containing node status string
 - events: dict of event counts as returned from event-counts endpoint
 - unreported_time: time since last report as string
Include status query in existing nodes method

The status information will be included if the parameter
`with_status=True`
Merge pull request #18 from juliushaertl/feature-nodestatus

Add with_status kwarg to ``nodes()``.