First stab at time machine using fixed two week expiration policy.

data/database.py

@@ -456,12 +456,14 @@ class RepositoryTag(BaseModel):
   name = CharField()
   image = ForeignKeyField(Image)
   repository = ForeignKeyField(Repository)
+  lifetime_start = DateTimeField(default=datetime.utcnow)
+  lifetime_end = DateTimeField(null=True)
 
   class Meta:
     database = db
     read_slaves = (read_slave,)
     indexes = (
-      (('repository', 'name'), True),
+      (('repository', 'name'), False),
     )