importer/twitter: follow up on little fixes

Change-Id: I6b7cfac3cf5893226016e819441f4cbbe458b4bf
search: allow searching by permanode location, not just file location.

Allows searching foursquare checkins by location.

Change-Id: Ib33d0b435a9bbc17cb860549b41d119f727197f0
search: temporarily hide foursquare.com:venue in results

Change-Id: If8d38ae4ba57852574b62991f612df6d59b7bbf5
index: add temporary hack for foursquare.com checkin times

Change-Id: Ia90097998ccb702b14c4634bb42be1e6387d61e8
Rewrite Picasa importer for the new Importer API

Reorganizes some common things with Foursquare.

Needs thorough review: several FIXMEs and TODOS included!

Change-Id: I0789f4732412aa559078ec2c6b6797925ab8cd05