Picasa: add importer.

Imports all images from all albums, and puts each image in its album.
Each album is linked under "Picasa (<username>)".
Tries to skip already imported files (filters by camliPath=album/filename),
but this does not seem to work as intended.
See Issue #391.

This uses ClientLogin, but works (I'm already importing my photos).

Change-Id: If00751671a429891c05bbe41f59c1421b2ff2de8