importer, picasa: move ExtendedOAuth2 out of pkg importer, fix picasa

The ExtendedOAuth2 code tried to be generic, but wasn't (had Picasa
comments?) and was only used by Picasa anyway.  We can move it later
if there are actually 2 users and it works for both.

At the same time, make Picasa actually work by writing a correct
IsAccountReady function.
Merge branch 'master' of https://camlistore.googlesource.com/camlistore