Drop use of gdata module in Picasa uploader

Doing without the gdata module has several advantages:
1/ It's not compatible with Python3
2/ It doesn't support OAuth2 - Google is phasing out OAuth1
3/ It doesn't appear to be maintained any more
4/ It's huge, as it provides access to many Google services (that we
don't need)
5/ It's one less thing for users to install

I'm now using the "requests" module to talk directly to Google Picasa
urls.