Auto-delete child objects when deleting Client or Probe object

although if there are a lot of readings, this will still suck...

rattail_tempmon/db/model.py

@@ -91,6 +91,7 @@ class Probe(Base):
         """,
         backref=orm.backref(
             'probes',
+            cascade='all, delete-orphan',
             doc="""
             List of probes connected to this client.
             """))
@@ -141,7 +142,10 @@ class Reading(Base):
         Probe,
         doc="""
         Reference to the tempmon probe which took this reading.
-        """)
+        """,
+        backref=orm.backref(
+            'readings',
+            cascade='all, delete-orphan'))
 
     taken = sa.Column(sa.DateTime(), nullable=False, default=datetime.datetime.utcnow)
     degrees_f = sa.Column(sa.Numeric(precision=7, scale=4), nullable=False)