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 (check based on album/filename).
See Issue #391.

Change-Id: If00751671a429891c05bbe41f59c1421b2ff2de8
Merge "Picasa: add importer."
website: add Picasaweb to 0.8 notes

Change-Id: I0de6dbfa506a9153a73c61e638ce4ddbfbfde702
more TODO

Change-Id: Ibb03612c1a06287bf781b96f478d4d627b159c84