importer/foursquare: remove unused oauthConfig

Change-Id: Ic663ad97ab41515d52e330544a546d58e8cbe583
importer/foursquare: few little tweaks, mostly logging.

Change-Id: Iee200c348ee89bf5ef37b4394d6320bcdcff3842
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