tasks: Add Google Tasks service support

Adds GDataTasks classes to libgdata:
 • GDataTasksService
 • GDataTasksQuery
 • GDataTasksTasklist
 • GDataTasksTask

This includes full documentation, but no test cases. No new dependencies
have been added to libgdata, as all the necessary ones were added with the
core JSON work.

https://bugzilla.gnome.org/show_bug.cgi?id=657539