Fix recursive delete of host

When host.delete(recursive=True) is called, it would
previously delete all services that contained host.host_name.

Now deletion is slightly smarter, and if there is a service
with service.host_name="host_a,host_b" and host_a is deleted,
then only the reference to host_a is deleted, not the whole service.
Attempting to fix a bug with trying to remove a scheduled downtime, in Nagios you cannot use comment_id, because that only removes the comment from the service status page.  We found if you instead go to Downtime in the Nagios UI the downtime_id is the id we want to remove.  This PR is an attempt to fix this problem.  Curious for feedback.
Assert that the service is on two hosts before host delete
Revert "Assert that the service is on two hosts before host delete"

This reverts commit aaad3720c04a7d58f25cbaf5a48f66a0e839e3fe.

.filter only returns the one result.
Merge pull request #162 from pynag/fix-recursive-delete-of-host

Fix recursive delete of host
Merge pull request #164 from codylane/bugfix_with_remove_schedule_downtime

Attempting to fix a bug with trying to remove a scheduled downtime, in N...