google: Add support for tasks

At the moment there is no separate switch for it because it is so
similar to calendars. We can consider adding a separate switch later
if there is need for it.

Access to the Google Tasks API was turned on in the Google APIs
console because the documentation suggests that it is the way to go.
However, some third-party websites [1] mention a scope string that can
be put in the sources. Given that Google's documentation has proven to
be unreliable in this regard we had added it too. For some context see:
2c93558da1e85bbf8c17715820344e7eb708f274 and
9182fb378614abb2ff0245fe71a753f22eaaa906

[1] https://discovery-check.appspot.com/

Fixes: https://bugzilla.gnome.org/693689
google: Update README

Sometimes the documentation does not mention the OAuth2 scopes that
need to be specified in the sources. Mention some of the third-party
sites that can be useful to overcome that.

Fixes: https://bugzilla.gnome.org/693689