[WIP] Delete taskqueue

[goru]

related #9

[goru]

I encountered limit of "Datastore Small Operations".
We have to think another solution or define rate limit of queue. :(

[goru]

I changed following points.

• use Query class. https://cloud.google.com/appengine/docs/python/datastore/queryclass
• don't specify parent when creating new entry (for support asynchronous delete)
• convert new style entry record if old style entry record is found when getting entry record.
• use TaskQueue when deleting entries of feed.
• add handler for getting / deleting single entry.
• add handler for converting old style to new style.
• add handler for checking status which feed has old style entries.