Don't use tasks to refresh feeds in the cron job.

Fixes #128
Reorder some functions.
Make functions for getting and putting UserInfos.
Use memcache for UserInfos.
Use memcache for FeedInfos.

Fixes #59

Conflicts:
	feedme/feed.go
	feedme/feedme.go
Use the memcache to store they keys of all feeds.

This allows us to avoid using a query to get the datastore keys for
all feeds during the half-hourly refresh task.

Fixes #135

Conflicts:
	feedme/feedme.go
Merge pull request #136 from eaburns/dev/memcache

Use memcache