Added ObjectDefinition.attribute_is_empty() for a quick way of telling if an attribute has no meaningful value
Small fix in attribute_is_empty() & test for attribute_is_empty()
Added Model.Service/Hostescalation, Model.Contactgroup.delete() plus Test for the later
Merge remote-tracking branch 'upstream/master'
Finished tests for ContactGroup.delete()
Added Contact.delete() and its tests, fixed contactgroup references=> attribute is called contact_groups in most objects!
Extended contact/-group delete tests to include more object types (hosts,contact/-groups) and their respective specialties
Extended Hostgroup.delete() to reflect discussion in https://github.com/pynag/pynag/issues/90
    removed workarounds for icinga issue 3900 as it is now fixed by attribute_removefield()
Added tests for Hostgroup.delete() (with recursive True/False) cleanup_related_items not tested separately
Extended Host.delete() to include cleanup of related escalations and dependencies as well as deletion of the same if recursive=True
- added tests for Host.delete()
Changed *.delete() to allow deleting of name-less objects (incl. indent levels)
Return self.delete() instead of returning None in the new host/contact delete()
Merge pull request #107 from gradecke/master

Better delete() implementation for {host,contact}[group]s